A Social Networking system (i.e Digg, BlueDot) reaches it's optimal usefulness when system activity exceeds a minimal value and is below a maximal value. This probably sounds pretty obvious on the surface. But it has some very significant implications for how much income a system can hope to generate from traffic. Just to tease this out a bit.
Some systems are useful on their own. For example, the most useful place to store bookmarks (to my mind) is online. So Social Bookmarking tools are useful even if you don't want to be social.
But for the Social element of these systems to be useful they need to achieve critcal mass across a wide range of topics (tags). This critical mass is different for each system. But needs to take into account each individual users level of participation. Critical mass is the systems minimal level of activity. For web 2.0 systems to survive they need to achieve this crital mass as quickly as possible.
A systems potential for growth is limited by two factors. The first is how well the sytem is organised. Not all systems are equally scalable. I suspect that the more input users have within a system the less scalable it is. For example, Google's search engine is theoretically infinitely scalable. Because users have very limited influence within the system. Which brings me to the second limiting factor. A system is limited by how open it is to abuse. In recent months theres been quite a bit of traffic around abuse within Digg. The evidence suggesting that Digg's top contributors were in fact employed to promote specific items within the system. This sort of press must have an impact on a systems potential growth. Obviously these two limiting factors are interdependant. The better a system is organised the less open it is to abuse. The way it detects and manages abuse is a measure of the quality of it's organisation.