A few minutes a day to see if I get any messages from family, friends, or forum members and staff on my forum since it is one of the ways I keep in touch with people I know. Also, Facebook pages for movies, anime, video games, etc are easier to follow than remembering to go to their official website, or blog with long website domain name, or searching the internet for the domain name.
I also use Facebook to promote my forum, blogs, and websites since Facebook has a lot of members, so trying to convince some FB members to visit my sites can lead to more members, and regular readers for my blogs rather than just relying on Search Engines, Word of mouth, Twitter, and other traffic refferal sources.
The last time I checked my Facebook account was months ago
The only social network I use actively is Twitter, and that's only cause my work requires it.
A few minutes a couple of times a day I spend now at least a few times per day. I used to spend a lot of time on Facebook when I first joined but I think like anything the novelty soon wears off.
I have my own facebook account and facebook page, the page needs to be updated daily, so I'm usually on facebook 4-5 hours a day to check up on things on my feed and all that. I usually don't acess facebook on my phone even though I have the app.