We may be in a bubble, but you wouldn’t necessarily know it considering the latest generation of internet IPOs.

Groupon’s stock is trading below its first day closing price, Zynga’s stock closed below its issue price when it debuted last week and the market didn’t seem too excited about the spin-off of TripAdvisor from Expedia.

So what gives? If everyone knows that the internet is the real deal, why aren’t these new issues selling like hotcakes? Here are five reasons.

The global economy is on the rocks.

The Eurozone crisis appears far from over, and there are plenty of other concerns weighing on the global economy. This, for obvious reasons, doesn’t create the most friendly environment for irrational exuberance, at least when it comes to tech stocks.

Companies are mature by the time they go public today.

Unlike the first .com boom, the tech/internet companies that are going public today are quite mature. This is arguably a good thing (we don’t need more Webvans thank you very much), but it also means that by the time a new issue hits Wall Street, much of the easy growth is behind a company, making it harder for investors, traders and speculators to drive the price up based on unrealistic future growth prospects.

There are real metrics.

Mature companies can be judged by more than just potential; they have real financials that can be evaluated. In the case of some of the highest profile IPOs this year, Groupon and Zynga, those financials raised a whole host of questions and concerns which cooled investor interest.

People don’t want to get fooled twice.

There may be slightly less partying like it’s 1999 today, but that doesn’t mean that parts of the tech industry aren’t in a bubble. Recognising this possibility, there’s a lot more skepticism about the companies going public. Is all of that skepticism well-placed? Perhaps not, but the desire not to be cheated again is a strong one that is probably keeping some from taking the latest crop of internet IPOs too seriously.

Retail investors have fled the stock market.

The Wall Street of today isn’t the Wall Street of a decade ago. Thanks to the global economic collapse of 2008, and the ensuing great recession, retail investors lost significant amounts of wealth. Many pulled out of the stock market and have been slow to return. Thus, huge demand from ‘mom and pop’ investors for hot internet stocks is simply not there, and the newest issues aren’t that useful to the vast majority of institutional investors and hedge funds.

The big question of course, is how all of this will affect the Facebook IPO, which is considered all but certain in 2012. On one hand, Facebook isn’t Zynga or Groupon, but on the other, it appears quite possible that Facebook, at the $100bn valuation it’s reportedly going to seek, will be closer to its peak (at least in the intermediate term) than not.

Facebook aside, the lesson for the rest of us is that even though it’s possible for technology companies to go public today, the IPO isn’t what it used to be. And that probably won’t change any time soon.