DA, it is, and pixiv maaaybe has chances to grow clsoe to that. tumblr i mostly only use for browsing dirty stuff, or otherwise hefty things. for posting works of mine, i also have both tumblr and facebook, but somehow both don't work that well, which is maybe so becuse of me having a kinda pretty limited attention currently...or 'attention span'?... not sure what'd rather apply here.
There's are several that I use, but none are what I'd call "favorites". They're just tools. You might as well ask which wrench is my favorite: a Crescent, a box end, or an open end. Each one does something better than the other two but they are tools, nothing more.
I have twitter, tumblr and google+ accounts, but I get the most mileage out of my Facebook account, mostly because most of the people I know and really want to keep in contact with are on it. It's certainly not perfect, even for a free service, but it's leagues better than myspace ever was and isn't as restrictive as a lot of the newer social sites that seem to cater to the lowest common attention span.
I have more than one and each serves a different purpose. At the moment, I said Tumblr first, than deviantArt and Facebook. My tumblr is mainly for fangirling over my favorite tv shows and other femslash related stuff (and related arists and blogs). DeviantArt is for Art : lots of quality, lots of choice, lots of inspiration, easy to manage and enter in contact with artists. While I keep my facebook more personal, with familly and closer friends. I also follow a couple of professional pages there that i find interesting, but not many.