Hi Piotr,
good topic indeed.

1. I think LinkedIn is quite different from those Facebooks and Twitters, because its a "job seeking" social network. From time to time, you search for some kind of job opportunity or you offer it to someone. And for that reason its good.
2. Surely they are not so perfect, while trying to convince you to give them your email and password "to reach more people" from your email list. This is a sh*tty forcement I really don't like. Your password should stay at your client, thats it. It could be leaked from someone etc.
3. For headhunters its great, because you simply focus on abilities and qualities, that people have (and fill in their profile). So if someone wants to be caught by a headhunter, one should fill the profile as much as possible.
   PS. 4. Crypto - I think even for crypto people its a good tool. People are having kind of more professional attitude in there, so you can feel "in better company" while talking with the same people. And also photos on the profiles lookes different than in fb (sometimes too much, but it comes with it).

ritxi