Lots of blog talk today about “bubbles,” in light of LinkedIn LNKD pricing its IPO at $45 per share.

To be clear, I do not believe that a single flotation is evidence for or against a broader trend. And LinkedIn clearly is a “real” business, even though it warns that its 2010 profitability will not be repeated in 2011. That said, I think many commentators are falling into a trap on their Internet bubble talk. Namely, if it doesn’t look exactly like 1999/2000, then it isn’t a bubble. This is ridiculous. Our most recent recession looked nothing like the 2002 or 1992 recessions, but it was still a recession. Not all bubbles are created equal: Some pop, some simply deflate.

My gut – and there’s nothing else to really rely on in such debates – is that we’re in the latter category. Many of the newer Internet companies are overvalued, and eventually will be worth less (but will continue on as viable businesses).

One counter-argument out there is that, until RenRen and LinkedIn, the expanded valuations were not being driven by Joe Investor. For example, Sarah Lacy wrote yesterday:

“As most people with common sense have argued, we’re not in an Internet bubble now, because the soaring valuations are mostly contained within the frothy insider ecosystem.”

I’m not quite sure why that matters. Are bubbles now defined by breadth of consequence? If millions of retail investors don’t get burned, does that mean there was no fire? Moreover, isn’t much of that frothy insider ecosystem (i.e., VCs) actually funded by teacher pension funds and the like?

Again, none of this means people won’t make great money off of these companies. Hell, people made great money off of investments in 1996-1998. But the one thing we’ve learned through all booms and busts is that there always are booms and busts. Cycles persist, even if some of them less nauseating than others…