Should I be using Spotify? And what's next?

I don't really know anything about it.  Is it the 2011 version of Napster?  And if that's the case, how is it legal?  I see posts on facebook where people have songs listed, and it looks like you can just click on them to listen.  But when I tried clicking, you have to sign up and add the facebook app, which is more than I wanted to do at the moment.  Is it awesome?  Am I missing out?  What does it do beyond facebook?  I really don't need any more crap on the computer to take up my time, I haven't even gotten around to Google+ yet.  And while we're on the subject, is that something I need to be doing?  Since facebook keeps sucking more and more, I'm hoping Google+ might be the next thing.

Friendster < Myspace < Facebook < ?

Whether it's Google+ or not, I feel like it's about time for whatever else is coming to start getting popular.  Also, was there anything before Friendster?  I don't think so.