Does posting papers on websites such as Academia.edu or ResearchGate count as "publishing" for the sake of future journal submission? That is, if I have uploaded my paper to Academia.edu, can I later submit it to a journal that wants only "previously unpublished" work?
Since nobody wrote it in an answer, I'll do it: the answer to your question depends on the journal, but in each field most journal would have about the same policy.
In some fields (e.g. mathematics, high energy physics), the answer is 'no': posting a preprint in Academia.edu, the arXiv, or on your web page does not prevent you to have your work published by a journal. Many publishers will even allow you to update your public preprint according to the referee's comments, only keeping the publisher-formated version behind a paywall (not all of them though: Oxford University Press has a very damaging policy in this regard).
In other fields (e.g. some humanities at least in some countries, chemistry) the answer is often 'yes': many journal would reject your paper right away on the ground that it already has been "published" in the sense of being made public. Even if they don't check, they may ask you to pledge that you did not published the material previously in that broad sense, and lying on these kind of issue may be devastating to a career.
In other circumstances, the answer may be more subtle. Some very prestigious magazines as Nature, Science, PNAS may ask for some publications that the authors keep them secret until the journals communicates about the work. This is to ensure maximum media coverage, but of course it concerns only the very small portion of academic works that is considered both as very important for the field, and of great interest of a general audience.
I wouldn't publish my papers on a website that everyone can see before it is actually published in a journal or a conference. Someone else may claim ownership and there is nothing you can do about it
In any case, assuming you did publish it on either website, you can still publish it in any journal or conference as long as you honor any copyright rules or guidelines.