Nowadays your site is not in fashion if you don't show at least a couple of share buttons like these:
Is this just fashion, or do people actually get something good out of it?
When I say "something good" I mostly mean something that you could measure, and not just the feeling that was good.
Maybe I can better explain with an example: did you notice (in some way) that many people clicked on those links to share your page/s on those web 2.0 social sites? And in such a case on which social networks did you see they mostly share your pages?
BTW I'm not talking about Google PR, i know all web 2.0 social sites use nofollow everywhere and even hidden links, so they are useless by themselves for PR.
According to this video, Google's Alter Ego says that they now use in some way data from social sites in ranking. If this is true, it's obvious that the Share on button for FB, Tweet, etc are definitely of some values.
But again my question is more about what you noticed in your real experience to be a direct benefit of adding those type of "Share On" links on your webisite? I.e. did you see more traffic coming in form FB, or some users who bought your products because of FB or Twitter? Or any other benefits?