It seems to me there is a fundamental difference between today's market and the dotcom bubble. Right now, sites with ridiculous valuations actually provide a great deal of value. Facebook has >500,000,000 people using it and makes a great deal of money. Groupon also makes a great deal of money. Google too.
Sure, it seems like we might be in a bubble, but at the same time things seem different.
But you have to remember that out of that bubble came a lot of companies that also provided value. Google came out of it (Google was from the last era, not this one). Akamai came out of it. VMWare came out of it.
The more things seem different, the more they're the same.
The big diff between now and then was that the first era was all about IPOs. Now its all about acquisition. I personally think the acquisition route is more ethical, since presumably the companies buying understand that your company may never be worth $X on its own, but strategically may be valuable. Whereas the IPO route seem like more of a Ponzi scheme.
Sure, it seems like we might be in a bubble, but at the same time things seem different.