My issue with this bubble is that there's no underlying. Instead of green technology, we see Zynga employing people who work full-time trying to exploit the same class of people that were targeted by the dreaded telemarketers of 10 years ago.
The late-90s bubble was frothier, with all the talk of a recession-proof "New Economy" (oddly, most people now long for the Old one) and the massively overvalued technology stocks. Even still, there was a real underlying. The Internet really was changing the way people do business in a rather permanent way. The reason for the bubble was that people overestimated how soon certain changes would arrive.
This bubble doesn't have much of an underlying. I'd even argue that there isn't one. Facebook is a decent product and has provided a lot of value to its users, but it's not as earth-shaking as the Internet, and these "Facebook's tapeworm" startups like Zynga are insults to the millions of people trying to build things that are real. Also, the current climate of extreme social openness (where people want to broadcast their careers and locations without thought to whether they really want people to have access to that data in 5 years) is going to end, and the "cool" social startups will either be dead, or successful and "uncool", within a few years.