Sweden has another interesting showcase with leading Lunarstorm and up-and-coming Playahead. Lunarstorm is one of the earliest social networking platforms aimed at a teen / young tween audience. The company appeared to have a tight grip on the Swedish market and over a million users (which is significant knowing that Sweden in total has 9M inhabitants). Clearly one would have thought that such a penetration of the target group precluded any competitor from entering.
However, Playahead still launched into this setting. I heard (unconfirmed) that Playahead has 450,000 active users, i.e. 40-50% of Lunarstorm's size. Playahead's positioning differentiated itself as a cooler, more partyoriented site. Correspondingly, it's user base is primarily in urban areas. So Playahead targeted a sub-segment of Lunarstorm's user base, and created a sizable community around it. When I recently had a a chat with Hjalmar Winbladh of Rebtel, I certainly got the sense the Playahead has momentum and buzz going its way, in the way of "if you are cool, you have to be on Playahead".
The Alexa chart below indicates that Playahead established itself firmly vis-a-vis Lunarstorm, and supports the relative sizes noted above. Interestingly, what we can't see is that Playahead ate into Lunarstorm, as Lunarstorm's traffic pattern didn't really change all that much as Playahead grew. What is interesting to note is that both sites don't grow all that much anymore, which suggests that both of them hit the ceiling in their respective target groups.

I'd submit that the bottom-line of this is that sub-segmenting a target group beats network effects. A tight offering designed specifically for a well-defined target group, can cut out a significant community even though the overall target group is already locked down by a larger social networking site. Secondly, like in "the real world" communities are subject to buzz, momentum, and coolness factor, which can turn audiences to one platform or the other. Certainly, I can relate to this from my non-web youth. One day, the coolest guys in our class showed up with Adidas sneakers. Then it took a while then everybody was wearing Adidas. That ticked off the coolest guys, so they moved on and started showing up in Converse Chucks. And on and on.
If I think about this from a venture capital point of view I think this is encouraging news in that we will probably see more new players in the social networking space. The things to think about is size-of-target-group (since ever sub-segmenting yields smaller and smaller target groups), and a potential faddishness of sites (particularly with younger target groups).

