I was invited to join the Hyves network recently and, though more out of work-related interest, I did. Wow, I'm actually quite shocked something this slow and badly designed can still be a success. As it's slightly related, check out Reader Poll: Are you burnt out on online sharing?
One thing I have become addicted to is Last.FM, but that's more because I love music and like talking to other people who share a similar taste in music. The latest site updates added concert listings into the mix, which made Upcoming.org which I previously used a lot also, kind of redundant for me. The main disadvantage of both systems for me is the lack of moderation. On the upcoming.org site this meant people could add metros on incorrect levels. For example, the Dutch provinces are present, but someone still added Amsterdam on the same level. And even Aruba. On Last.FM you have similar situations were people add gigs that are already present in the listing. Not sure how to solve this, but it does annoy me.