Is there really no alternative? What is it that makes it compulsory to use?
I have a very out of date profile, but rarely use it. For some reason I don't seem to get spammed. I don't see any use for it, so I don't visit.
However my most interesting recent usage of LinkedIn was signing up to pay for Inmail in order to be able to message a few high level executives at DHL to complain when a delivery was stuck in limbo. One day later and two Senior VPs (out of 3 I messaged) in different offices had told their staff to sort it and I was Cc:'d on a flurry of message exchanges that started or ended with pointing out that "SVP so-and-so wants to be kept in the loop".
Two days later the package that DHL for a month had told me they didn't have (and had told the sender was on it's way back to them), and couldn't have corrected the delivery address on even they did have it (it was accidentally - my own fault - sent to our old office) turned out to be sitting in a depot 5 minutes away, and was promptly delivered (to our new office).
That to me highlights a benefit of LinkedIn: Everyone can get access to relatively high level people, in part because the barrier to hassling them is high enough that it's not massively abused: If you're not in their immediate network you can contact them directly only if you've paid, and only quite few at the time. While it's annoying to have to pay, it's better than not being able to reach these people on occasion.
I'm going to consider using InMail that way whenever someone annoys me enough to be worth the cost of another month of InMail subscription from now on given how well it worked with my DHL problem.
Presumably most people want to do the right thing for their customers, but frontline staff often don't have authority, and soon gives up escalating systemic complaints (or even gets punished if the continue to bring it up), so if you don't get help right away the solution is often to find a way of bypassing the layer(s) that are powerless or have given up trying, and you find people that often do care deeply when they get to hear about such problems, but rarely do hear.
The moment something like this becomes common, these people will retreat from LinkedIn, or LinkedIn will take measures to protect them (win-win for LinkedIn: Let extra busy people make it more expensive to reach them)
I've used Twitter too, as well as my blog + Twitter. A few years ago I'd problem cancelling my cable subscription, and ended up getting a call from an assistant to the CEO of Virgin Media on the Saturday morning following a posting on the Friday. But in the case of DHL, Twitter too was a dead end - they seem to not care much about their Twitter feed.
For me, it's because not being part of the network is worse than being part of the network. LinkedIn is probably more prevalent in the business world than even having a Facebook or Twitter account, so to me it would look odd if I were not at least findable there, even if I don't maintain my profile.
It would be very easy to build an alternative. The hard part would be amassing the critical mass of recruiters. For a while Forrst was on the right track but it no longer exists.
Now, the story of how I got my current job: met guy at local Meetup, talked to him a bit and found out the company was hiring, and he would mention my name to recruiter. So, I add him on linkedIn, but get no call whatsoever from any recruiter. I see that there's a company recruiter on his connections and add her; explaining that I had met so and so at a meetup and there is an opening for a person with my skills so would she be kind enough to set me an interview? It takes some time, but I do get the interview, clear it, and all the other interviews as well.
Didn't get the job -- at least not yet :) But it was an interesting exercise.
e.g. I already sell a product to lawyer John who practices speciality X. With LinkedIn it's easy to find all the people who look like John, and then try to pitch them on my product.