Twitter – I have an account. Logged in a couple of times but doesn’t do anything for me. I participated in an online course that had a systems for collecting tweets about the course and putting them into an email along with other information about the course. I occasionally found links to articles of interest, so I guess it is good for something.
LinkedIn, Facebook – have accounts, very occasionally access them, usually for something specific – when my niece was in Afghanistan, I knew she was ok because she posted something most days. Found some friends we had lost contact with 10 years ago. Not much else.
Generally, these are potential liabilities to personal privacy. You aren’t their customer – the advertisers are and you have no idea what lengths they will go to to keep their customers happy.