It seems there are new social tools created everyday. So I had a few questions:
- How long does it take for these networks to reach critical mass?
- What happens when the market is saturated?
- How many networks are people willing to invest time into?
- What's next for them?
To answer the first question I went to Google trends and plotted MySpace versus Facebook. 
From this it looks like myspace has peaked and facebook is nearly as popular now. Next I went to alexia.com which ranks sites. Here are the top 20 sites:
| Rank | Change | Web Site |
| --- | --- | :-- |
| 1 | (none) | yahoo.com |
| 2 | 
1 | google.com |
| 3 | 1 | live.com |
| 4 | 1 | youtube.com |
| 5 | 
3 | msn.com |
| 6 | (none) | myspace.com |
| 7 | 4 | facebook.com |
| 8 | 1 | wikipedia.org |
| 9 | 1 | hi5.com |
| 10 | 2 | orkut.com |
| 11 | 2 | rapidshare.com |
| 12 | 2 | blogger.com |
| 13 | 2 | megaupload.com |
| 14 | 3 | friendster.com |
| 15 | 1 | yahoo.co.jp |
| 16 | 7 | baidu.com |
| 17 | 2 | fotolog.net |
| 18 | 1 | microsoft.com |
| 19 | 4 | google.fr |
| 20 | 7 | qq.com |
Of the top 20 - Myspace, Freindster, facebook, orkut, and hi5 are all social networks (a collection of personal profiles linked together and competing for attention). The rest of the list is search engines, webhosts (which rank high because a lot of websites have that domain like rapidshare.com and blogger.com), and wikipedia - the king of user created content. Baidu.com is the top Chinese search engine.
Here is the one year trend for social sites on alexa: 
One thing that is clear about the top 20 sites - user generated content is definitely king. Not a single site other than microsoft.com pay for content - they all harvest, search, sort, rank, and enable the general public to do it for them.
So getting back to social networks, how long can these things last, I think once they hit the level myspace and face book are currently, they have to innovate quickly or die. Like shopping malls in the real world, the crowds come for a while but are quickly lured away by brighter lights. Plus, a lot of users begin to get turned off when a system is seen as too main stream and want the cool factor of the next great thing. In any case there are plenty of options for users to try. While I do have profiles at Linkedin and facebook, I don't update them. If I'm going to generate content, I want more control over it then those sites allow. Also, I want to benefit from the advertising.