I should clarify what I meant by "decay"--services can continue to grow, but if the quality (defined loosely) goes below a certain point, I think it's hard to make them into really valuable businesses.
The quality of the conversation has gotten worse and worse, things are targeted towards the lowest common denominator.
You need to go to specific subreddits or really dig around in the shit to find the gems.
The flip side is that it generates a ton of content. To the signal to noise is horrible, but in aggregate there is more interesting stuff going on then on slashdot or wherever
Social sites like Facebook are fine for social interaction, but they are missing a boat when they don't give individual contexts to different situations, all pertaining to a single person. I'd rather be anonymous in one context and not anonymous in another, but Facebook doesn't provide such feature. Sites like Reddit on the other hand are at the opposite spectrum. You are technically anonymous across the board.
The balance between the two is that of context. Give people the ability to set their own context and that would be the ultimately best social Internet.