I enjoy using it as a statistics aggregator as well. Last.fm definitely gets credit for capturing the primal sense of achievement with scrobbling. People I know don't really use it as a social network other than friending each other, but even that is sufficient because it allows you to peek into their current musical tastes. It's sort of elegant how Last.fm's experience is pretty much just collecting data and displaying it nicely; no need for apps, uploading user content, or location-based gimmicks. Just listening to your music library is generating content enough.
As far as listening goes, most people I know use Pandora.