For me the Twitter use case is to keep myself stimulated while working remotely by interactions with other people in my field, catch breaking news as it happens (e.g. blog articles on new tech), and to promote work that I have done (e.g. blog articles I have written). So when I am working actively on a project, I probably check 4-6 times a day. In between I probably check 2 times / week. I also have used monitoring tools like awe.sm to see how many people re-tweet things that I tweet out, among other things to make sure I'm providing value to people that are following me (i.e. high signal to noise ratio). I also use twitter to answer questions that I can't find the answers to elsewhere -- generally very technical questions.

Facebook is a tricky one, since it promotes addictive "stalking" behavior. I've attempted to use it primarily for messaging and to hide most people who aren't in my immediate vicinity, thinking that I probably don't need to know what they are doing. Still, I am a part of "liking" tribes where there is an implicit economy of trading likes on something and receiving likes back.

G+ I only use when I get annoying messages from Google that someone has asked me a question or added me to some circle. Other than that I avoid it like the plague -- this despite being an enthusiastic adopter in the early days.

LinkedIn I use probably once a month, except in the occasional moments where I am actively networking. I get very little value here since I am not usually looking for work and the technical folks I interact with are available on Twitter (or, at worse, email).