I find it annoying in general
Loads of people want you on their profiles so they look good or are popular, or trying and sell you something and shine you up at the same time mostly pretty young Chinese girls working in marketing.
Latest thing is endorsing peoples skills, people have endorsed me and they don't even know me.....
Public Groups are mostly full of idiots imo, I am in some invite only groups where I personally know the companies and people involved I have dropped form the less fussy groups as I can take the crap spouted by graduate industry noobs

Basically if you have time to sift through the sh!te it can be useful.
It is good to research who is doing business with who but you cant rely on the validity of how connections between people are made or embellished.
And as said before its full of recruitment agencies trawling for candidates, I get 10/15 hits a week on jobs around the world €90,000 was offered to me to go to Ireland before xmas.
On the whole its a face book for work which takes you away from doing more productive things