I'm a tumblr user (although I am a bit older than the usual tumblr demographic), and a member of SMblr, but my vote goes to Facebook. For me personally, SMblr is a great resource but provides very little that SMNet doesn't do- a network of SMs sharing advice and experiences. The tumblr format makes everything more casual & personal than SMNet- lots more memes, microblogging, personal stories, but in terms of overall usefulness? Facebook wins.
EVERYONE has a Facebook, very few people won't "friend" their coworkers/castmates, it's easy to create a group, and it's much easier to keep in touch with actors that, for instance, I haven't worked with in a year or more but are in a show I'm going to see. When my phone decided not to receive texts a few weeks ago? Posted a status about it on Facebook and got messages on there instead of texts. When I have extra comps to my shows, I post it on Facebook.
It's a much better local tool than tumblr (in my opinion)- I think I only know one other DC-area SM on tumblr (hi @leastlikely!) but I'm friends on facebook with every SM I've worked with since I moved to DC, along with most of the actors, directors, designers, etc. That's where I'm going to go if I want to catch up with someone I haven't seen in a while- see what they're working on, oh you're down the street from my show, let's go get drinks!