Among other things: invasion of privacy, farming of users' data to advertisers, potential to lure child predators and other criminals, wasting time (often why schools/governments block access) Here's...
No, because it's not technically supposed to be a social networking website. This is a Q&A website.
Yes. YouTube can be classified as a social networking website, as it offers users social networking-type services (among other types of service).
Facebook, MySpace, and Twitter are three of the most popular.