Not really. If you feel no peer pressure to sign up, great!
For me, I was missing out on events when classmates and even teachers started communicating via Facebook. These days it has died down again in favor of Telegram so I could technically deactivate the account, but I'm keeping it for convenience. Recently saw someone's wedding being announced there, not something I want to be missing out on. I'd have caught on eventually, but this is easier.
It depends how often I check it. If I have lots of stuff I want to do, I might not check it for a month. If I'm bored a lot (i.e. when school begins again after the holidays), I'll probably be on it before Monday morning is out, and be checking it every other day from thereon.
Linkedin:
Depends on your situation and business I'd guess, but as someone with not too much experience with Linkedin, I don't know. I've got it and I actively make connections, but I don't really use it other than that. More of a backup thing.