Are the markets getting frothy again?

It's hard to pick between the bubbles-are-bad and bubbles-are-O.K. camps is that bubbles aren't all alike. The best ones create assets whose value survives the crash. The Apollo program that put people on the moon, only to lose public support in the 1970s, was a "social bubble" in which over-optimism advanced science. Bad bubbles generate worthless assets such as exurban housing subdivisions that are taken over by squatters and mold. Other bubbles don't produce any supply response at all. The only impact of China's new mania for old wine—one bottle went for nearly $233,000 last year—is to transfer wealth to whoever was lucky enough to own the bottles before the Chinese got interested.
When the tech sector gets bubbly, consumers are often the biggest beneficiaries, notes Harvard economist Edward Glaeser. 
Glaeser says:

Because investors fund ideas that help the general public, from wireless communications to solid-state data storage to the Internet. So it was in the 19th century with the railroad boom. Today's speculation in tech is concentrated in social networking. 

The question is whether the new investments will live up to the greatest hits—and productive busts—of Silicon Valley's past.