I am trying academia.edu. So far it seems like it has its good and bad elements, like most anything else. My reasoning is that academia.edu can be a good place for an online public "filing cabinet" of sorts: for papers in progress, talks, open access publications, etc. Why not just use one's own personal website as a place to post papers, or link to google docs, you might ask?
To its credit, academia.edu has a very good search option and can serve as a "central hub" for those posting their research, in whatever form. It can be downright difficult to navigate personal websites (or even find them), and so by intuitively just cruising around academia.edu not only do you find interesting papers, but you can find scholars who have a web presence, there, for you to follow. Those presences link to other closely related people, and so on.
I also like how you can look through papers as "works in progress." I am in a minority thinking this way, as many warn against posting unfinished work. However I think its good to see that ideas can change in real time given productive dialogue with scholars at other places (blogs, email exchanges, or even in person at conferences or whatever). If a piece is going in for submission to a journal, I see nothing wrong with posting it in germ form to have others provide feedback. Of course, its always wise to post a link to the finished product, too, in order to see how things turned out. You can edit the title so that the paper doesn't appear in a google search during the peer-review process.
It seems good as well that the site - like blogs - provides a democratic voice for publication. Open access is often touted as free and democratic publication, which it is, but of those open access venues there are still political measures in play that can keep good papers from appearing, even in a free form. So like blogs, academia.edu gives those who would otherwise be silenced a voice. If you have something to say and it isn't yet quite fit for publication, *or* some just don't want it to be published for authoritarian reasons, you have a place to publish the paper where others will easily be able to search for it and find it (and sometimes folks could find it more easily there on academia.edu than they would by hearing about the open access journal in which it would have appeared). So unless the open access journal is supremely well established, you may get more eyes on your essay by putting it on academia.edu, which is interesting.
So I think its good that using search terms etc. you can essentially have a very broad range of open access that isn't stifled by some of the current problems in open access publishing through typical channels of journals. And there seems to be better search features in place that may trump the promotion efforts of just one journal. On the other hand, you lose the rub of having the piece appear in a journal, as well as lose an official peer review process. So there are drawbacks as well, but they are the drawbacks of not publishing something "official" (and I am not sure what that means).
The quality of papers varies widely, of course. But this problem can be self-correcting I think. Various scholars have their work on display. Once you find someone whose interests match your own, you can just determine if you'd like to keep coming back or not.
The downside is that it doesn't seem that folks frequently use academia.edu. Maybe a few hours a month, if even that. That is because not many are out there cranking out papers left and right. But email updates (I am assuming) *should* notify you when someone you are following has uploaded a paper. We'll see about that.
Another downside is "interests." At first it appeared as if I should limit my research interests to categories and labels that I would put on a CV. But that's actually harmful, because I want to know more about my, say, three specializations and may want to know about an array of historical figures or debates in current topics. I am noticing that the average user lists anywhere between 20-40 research interests (mine has ballooned up to 50-60, but I've included a dozen or so historical and contemporary philosophers in my interests that I'd like to read about in the feed that pops up). I judge research interests by the top three interests which are visible without having to click on the "more" tab. If anything, the research interests box "paints a picture" with many, may tags where you get a general impression of what the scholar is interested in, with those first three being the most pointed description because that is what is immediately visible.
In the end, my general impression is that academia.edu best serves as a public filing cabinet and research hub where you can find other scholars and papers who have similar interests. That tool just isn't available through google, where scholars may not even have a personal webpage or blog (in fact most don't) where you are dependent upon the whims of the person putting up the paper on a school server or google docs or whatnot. It seems like a good way just to sort through tags and search terms and find similar research interests, or those with whom you might even like to correspond, network. (I understand that I haven't even touched the "open access question" - that is, what of putting published papers there, as a repository, etc.)
Ah, this brings me to my last point. The difference between a blog, academia.edu, and PhilPapers (as I am reasoning about this) for me is as follows: blogs are often "scanned," rather than read. It doesn't work to try to post formal academic essay fit for journal publication as a blog post.
This seems true because as new posts arrive, a post which you spent so much time on creating is knocked down the list and disappears in a day or two. If someone reads your blog through a feed they may even miss it. Blog posts lack permanence in that way. The blog "mini treatise" is just such a waste of effort, even if you are able to pull it off, because the amount of labor you put it doesn't match the amount of labor people invest into it. And this happens by the very nature of a blog post. I save personal opinions, diary-like entries, recommendations, or quick thoughts for the blog. Philosophical posts in rough form can appear there too, of course. But I think just the fact that something is a "paper" tends to slow down the reading mind. And academia.edu is just the place for that. Papers or philosophical pieces "stick" - they aren't disappearing anywhere with time and people can know to look for them there. Once things are "officially" published and set in stone then one can list them on PhilPapers.
So I would use PhilPapers as my personal research bibliography of things I've published or are due to be published, and academia.edu as a filing cabinet and network hub or sorts.
In closing, I also want to say that I am not completely sold on academia.edu. For me this is a trial - like I tried with Twitter - and I may just delete the account if its not working for me. But that's what's nice about these platforms, you can just stick with "what works."