I have a linkedin account from a brief period when I worked "in industry". When I subsequently did two (pure/theory) postdocs, I updated my details to reflect that fact. I haven't actively used it to look for jobs or connections much, but it seems unlikely to be useful to me. Linkedin is too clogged up with other stuff, there are more direct ways to eg advertise jobs in academia. On the other hand Vaclav Chvatal sent me a (probably automated) message on there about his new book, which is about Erdos, nothing to do with industry or anything. So at least someone out there (maybe Chvatal, maybe some literary agent or something) thinks it might be useful...