Personal web page vs Academia.edu vs ResearchGate
As soon as you have even a single preprint, people will begin searching online to find out who you are and what else you have done, so you must have a web page.  It doesn't have to be elaborate, and it's enough to start with a few lines of professional contact information and a list of links to papers, but you have to have something.

I think a generic web page looks more professional than one created using a social networking site, but perhaps that's because I'm old.  However, there is one absolutely critical issue: the page must allow visitors to download any content without logging in.  At least one of the social sites lets visitors view papers on the site, but insists that you create an account if you want to download anything.  This is terrible!  In my experience, nobody's going to create an account unless they really, really want that paper, and either way they are going to be unhappy at the imposition.  Offering access to papers and then harassing anyone who tries to download them leaves a very bad impression.