LinkedIn is the most pointless fucking thing ever. It's incredibly basic - you just add people, that's it. The community discussions and news feeds are all terrible. You can't even customize it to be less terrible, unlike facebook which can at least be tailored to be as intrusive as you'd like.
The only value of LinkedIn is as a virtual wallet of business cards. A lot of people change jobs, so it has the advantage of being independent of company email addresses.
The colleagues I respect the most for their technical abilities don't use LinkedIn, and that includes other geologists, engineers, commercial analysts, economists etc... When those people have moved on and found jobs, it's because of existing relationships outside of LinkedIn.
The only outlier is HR - that function exists solely to create work for themselves and create shitty presentations with shitty clipart and buzzwords so of course they fucking love LinkedIn. LinkedIn is also a fantastic tool for headhunters who suck at their job and want to advertise their uselessness by PM-ing all potential clients and personally alerting them to the fact that they suck at headhunting.