Very rare, generally because there wouldn't be much to put on it. I've also found that the only people to have a LinkedIn are more professional people with quite a fair bit of experience in previous roles and several qualifications (just what I've noticed). Most people will set one up at university or during an apprenticeship, when they've got more to put on it. I plan on setting one up at university, when I can add GCSEs/A-Levels as well as internships or societies etc.
I'm sure most people use it just to be nosy and see where colleagues/friends previously worked!