Personally, it's a great business card replacement. If I'm at networking events, conferences, etc. I also prefer to have my personal (Facebook) and business (LinkedIn) networks separate since I use them for very different purposes. On occasion, I've also leveraged groups to get specific advice I wasn't easily able to find elsewhere.
In running my company, LinkedIn also creates a quick talent pipeline and allows candidates to get a rough understanding of who will be interviewing them so we don't need to spend precious interview time answering their questions about our backgrounds.
I don't know of any replacements that do all of these things under one roof reasonable well. I do know of a few individual tools that offer incremental improvements over what LinkedIn offers, particularly in the area of finding qualified candidates, but in other areas (ex. digital business card) LinkedIn is the only real game in town.
Yes because yes. When I last looked for a job, I poked around LinkedIn to see who was working at companies that I found interesting, got a former co-worker to refer me for his new place, and got a job there.
Yes. Multiple times.
I ignore recruiters if I even get a hint that they're not employed directly by the company they're purporting to be with. It's become less of an issue within the last year or so.
The other social networks eventually followed but LinkedIn just gave me zero value from day one. It was nothing but recruiter spam. Do people actually get work and jobs out of LinkedIn? Or is is just a way to keep up with what old coworkers are doing? I frankly do not get it.