I have been a member of LinkedIn probably since 2011. Following the 2010 23things, I blogged that I'd seen no reason to join the site. I now have 169 connections, but I suspect I will never view this site with the same enthusiasm as Twitter.
For me, the best use of LinkedIn, as of Academia.edu, is for tracking people down and making contact with them -- a boon when acknowledging gifts to the Haddon Library from people who aren't based in Cambridge, or, conversely, seeking to levy fines from people who've left.
Following the promptings from 23 Research Things, I have polished up my LinkedIn profile, adding information mainly about my volunteering with AgeUK; joined an Open Access group which didn't seem to have the expected Cambridge people in it; and turned the activity alerts off.
Not the keenest endorsement, I know. But keeping a sober profile on LinkedIn will be useful in sobering situations.