According to several traffic measurement companies (Quantcast, etc.), Twitter's traffic is exploding, followed by LinkedIn, Facebook and other networks. Once you factor in and remove spammers, growth numbers look much more modest.
From our experience, we see very strong growth on Twitter, but after removing spammers, the growth is close to 0. LinkedIn shows a strong growth too, smaller than Twitter, but after removing spammers, LinkedIn growth appears much stronger than Twitter.
Facebook is not working that well with us - growth is very modest because Facebook members are not interested in business. However, we have found very little spam on Facebook, compared with Twitter and LinkedIn. For us, LinkedIn is definitely the winner, after factoring in spam and automated traffic that is not accounted for by Internet traffic measuring companies. Ning, which provides strong growth to AnalyticBridge, also brings its share of spammers - more than LinkedIn, but much less than Twitter.
I believe that eventually Twitter will clean its traffic. But for now, there is a great opportunity for start up entrepreneurs: creating a tool that allows Twitter, LinkedIn, Facebook and Ning network creators to automatically flag members that have a bad spam score, and allows administrator to automatically block / remove / ban members with very high spam scores.