The people tech types forget about are the late adopters.
The internet pre 2000 was built by tech types / geeks but the new internet post 2005 is going to be decided by late adopters and "normal" people.
These people don't do fads and won't change technology on a whim. There are 500 million Windows XP computers out there and late adopters are quite happy using them! (Okay, throw in a few million mission critical XP business systems too but it's still a high figure)
IMHO there was a time frame circa 2005 when broadband started to come in and the average Joe decided that the internet wasn't this geek toy but a thing they should have. Any big internet giant at this time was gonna hit the crest of the late adopter wave.
Google hit this wave with search and email.
Facebook hit this wave with social networking.
Hotmail hit the wave with webmail.
Yahoo hit it with email and the tonne of web properties it owns.
AOL pretty much the same as Yahoo.
Twitter hit it with more newsy / celeb / hobby social networking.
Bing (Or Windows) hit it because many late adopters don't know what Firefox is!
The major problem a new network has now is where does your critical mass come from? Sure if you're pintrest or tumlr you can get the kids involved but the minute they reach late university age they will grow up and start using Facebook to keep in touch with family / friends at the same time.
You can also try to get a certain demographic involved but this is always gonna be hit and miss especially when the world is becoming more globalised. I can see a world perhaps where some people will belong to a smaller social network ie: An African American network or Gay network etc and also run a Facebook account too.
Facebook is way to mainstream and too intertwined with late adopters lives to fail now, it's basically the ideal cash cow. (Just as Google search is too)
Personally I see Facebook, Google, Yahoo and Twitter hanging around for the next 20 years. Unless something completely earth shattering happens to the internet in that time the status quo is gonna sit in the middle of the map and spawn camp startups like a pro!