The first place to check is TweetScan, which scans what twitterers are saying. Many twitterers are early adopters of these networks and may mention if it's worth joining.
Secondly, try googling the name of the site and looking down the results. Often you can find out what people are thinking from various blog postings about the service.
Be very careful about which online services you join, how much information you share and who you invite. Several, including the popular Hi5, have a reputation for spamming all the contacts in your address book with invites. As a rule of thumb, use the site for a few months to judge whether it's worth it before inviting everyone in your address book.
In my own case I stick firmly with LinkedIn and Facebook. There are many pretenders out there, some with much better features, but it's the quality of the network that counts in the end. Right now, only the big hitters can attract all the people in my networking circle, and Affluence.org and Naymz just don't cut the mustard.






