I don't use LinkedIn that much, mostly because I think people overreach and use the wrong standard.
A common problem in etiquette is when people confuse social with business. And they insist that coworkers go out for drinks; or they feel they'd be rude to switch dentists or hairdressers.
LinkedIn reflects some of that confusion. The aforementioned endorsement of my [work duty] skills by someone I went to high school with; she knows where I've worked, but she's never, ever seen me do my job. And in fact, though it's not "rude," I think her endorsement *is* a "bad form" etiquette violation.
I've written recommendations for other people--like my real-life recommendations, they're pretty detailed and specific, therefore long. I think they're pretty convincing, and I think the chances are good that *if* someone was on the fence about a hire, my words would tip them over into choosing my colleague.
I won't "link" with relatives or friends. I keep it for colleagues I've worked with only. I won't link with people who've applied to me for freelance work but *haven't* been hired. Basically, if I know you well enough that I'd be happy to answer Q's for you about a place you think you might want to work, or would be glad to call a friend on your behalf for info or to say, "he's really pleasant to work with, and when we worked together, I always found him helpful, even though I didn't see his work day-to-day close"? Then I'll link.
Partly that's because I just don't want to get into a far-flung web of connections. I don't want to do those sorts of things for just anybody--I don't have time.