Twitter is a joke. And FaceBook is not a "six billion dollar company". And "millions of users" who are not paying for your services do not translate into sustainable business just because of the "millions" word.Revenue is revenue. I don't know who's saying Facebook is worth $6 billion, and I think it's overhyped too, but I'll bet Facebook is cashflow positive. Twitter's a joke? If it never becomes more than it is, then quite possibly. But perhaps its engineers plan for more than what it is today. And quite possibly, surrounding technology (cell phones, what-have-you) will advance in ways that will make what Twitter is suggesting more interesting.
Overhyping != a bubble. Companies making stuff no one wants != a bubble.
A "bubble" refers to a stock market bubble. We are not in a stock market bubble. If startups make things people don't want, then they may fail. Or better, they'll learn from their mistakes, improve themselves, and make things people want.