Facebook: Not really... It's only there for "security reasons" and, quite frankly, it's now only being mainly used for "casual" badge-hunthing... (The again, I do have a Facebook for my Youtube channel, but I digress...)
Twitter: Unless I do something on Youtube, which triggers its "auto-post" function to work or I decide to participate in @Midnight's "Hashtag Wars", my twitter account is there to look pretty, I guess...
Tumblr: To be honest, outside of porn, more porn, actual porn art, haikus, music [at one point or another], certain "role-playing ask-type" blogs, and "Rule 63" as of recently, I keep forgetting I have a Tumblr most of the time unless I'm in the mood for some porn or something like that...
Other than that, I consider going on forums a means of going on something "social media"-related in some way, shape, or form...
