LinkedIn tells your boss that you're looking for a new job.
I mean, everyone is looking for a new job, either explicitly or just keeping an eye out for opportunities. But there's no reason to give the bosses any reason to think you might jump ship.
Anyway, I had a LinkedIn account for a couple of months back when it was a new thing. The primary results:
a) (actual) friends "recommending" me for particular skills I have but they have no way to support their recommendations. Thanks, guys, I know you mean well, but ...
b) a never-ending stream of salesdroids calling, asking if I do Skill X (no, I don't, didn't you bother reading the list of skills, oh, of course you didn't), and if not, can you connect me to the people who do? It got to the point where I (and others in the office) had the receptionist ask the caller to state their business. So the person trying to sell MCAD software (for example) would be told, "That person is not involved in that" and the discussion would end and the call would never get through to us.
c) the never-ending stream of recruiters calling on an office phone number during business hours. (All calls in go through the main line and if you don't have the extension you have to go through the receptionist.) And literally every single one of those callers (before they, too, got shut down) was pushing a sales-engineering job, never a real design job. The first few times it was amusing: "didn't you even read the skills list? what makes you think I want a sales job?"
Yeah, I realize that salesdroids and recruiters have no qualms about fishing expeditions and they're unafraid of rejection and they don't care whether the person they're calling is even suited to the job being offered.
And LinkedIn doesn't care, either.
As for Facebook: yeah, a lot of you are all really proud that you've never used it and blah blah blah. But I find that it does serve its original function* of helping to stay in contact friends (actual people who know each other) and relatives (if you care about them) who may live all over the country and whom you actually see ... rarely. So it's good to see what they're doing, what their kids are doing, what their cats are doing, all of that. I have a lot of friends who were in touring bands (or were on tour or venue crew) back in the day, and it's nice to see what they're doing and hear their new stuff, and if they're going on tour. When I have shows I post links to them and friends get in touch and we get dinner and hang, which is part of the reason to keep getting in the van after all these years.
It all works well enough if you stick to a few simple rules. First and foremost, ignore or delete friend requests from people you don't actually know in real life. Second, ignore trolling. Third, if that person from high school turns out to have become a raving lunatic, don't even engage, just block the person. Fourth, go through the privacy settings and make all posts and all photos friends-only. (This is important, so the potential next employer sees only a photo and location information and not much else.) Fifth, don't join any FB "groups," as that's part of the data-mining operation. Sixth, disable the "personalized ads," and always browse in Private Mode through a VPN which limits the tracking. (Your ad feed will be full of nonsense.) Seven, never use the FB apps, only the private-mode browser.
* regarding "original function," I realize that Zuck and his douchy Harvard friends created FB as a way to basically rate college girls on their looks.