I came across this post from the GeekBox blog (excerpt):
With anything that grows at an exponential rate, there is a tipping point at which it takes off. Tribe.net hasn't quite gotten there yet, but I'm starting to see signs of something new and different. As I was trying to point out before, it comes down to subtleties.
In this particular case, I suddenly noticed the utility and novelty of the simple fact that you can see to which tribes other people belong. It may not seem like much, but it's actually a big deal. Part of what made Napster (and its successors) useful as a means of discovering new music was the ability to browse other people's music collections. Chances are, if someone's collection contained music that you really really liked and some that you'd never heard of before, the unknown stuff was worth checking out. (c.f. my post on Echocloud)
People on Tribe generally don't list enough musicians, authors, or films to make this sort of cross-pollination work as effectively as it does on a P2P network, but they do list every public tribe to which they belong. Tribes are growing in all different directions, with little centralized planning. The directory is becoming unwieldy, which at first seemed really irritating to me. However, if you poke around your friends and their friends' profiles, chances are you will discover some interesting sounding new tribes amongst ones you're familiar with.