The past year or two could have been the start of another head-scratch-inducing bubble on the stock market. Groupon (Nasdaq: GRPN), LinkedIn (NYSE: LNKD), Zynga (Nasdaq: ZNGA), and Pandora hit the market, followed by the IPO everyone had been waiting years for -- Facebook (Nasdaq: FB). It looked a lot like the late 1990s, as popular companies with questionable revenue and profit potential came to market with expectations that were through the roof, a recipe for disaster.

But something interesting happened. Instead of foaming at the mouth over these hot names, investors approached them with remarkable restraint. Facebook has been the most high-profile flop, falling 50% in just a few months as a public company, something that seemed impossible a year or two ago. The most popular social network's revenue growth and profitability has been the big question, such a mature question for a market that once cared more about clicks than profits.

The bubbles of old
It hasn't always been this way. In the 1990s, the Internet bubble tried to change everything we knew about investing, focusing on clicks and eyeballs instead of traditional metrics like revenue and profit. Even the great Warren Buffett was questioned for missing out on the Internet boom, preferring to focus on stodgy companies like Coca-Cola, Gillette, and American Express.

In the 2000s, the housing market was all the rage. Miami, New York, wherever -- just buy, buy, buy! It's a real asset; it will never go down (we thought). Banks pushed formerly unheard-of products like ARMs and interest-only loans, and turned around and sold complex structured products and derivatives on such assets to their financial clients.

We all know how those two bubbles ended, and for some reason, I thought this batch of big-name IPOs would be the same.

Signs of maturity
Every once in a while there are signs that give me hope that the stock market is more than just a speculative machine and that there's some sort of fundamental valuation behind it. The performance of the IPOs above helped, but last week the sign came from a "disappointing" IPO by English soccer club Manchester United (NYSE: MANU).

Sports franchises are notoriously popular ways for rich people to show off their wealth, and Manchester United owner Malcolm Glazer certainly loves his sports investments. But sports clubs have a history of poor financial performance and don't generate the kind of profit the market appears to be valuing today. MLB has dealt with the bankruptcy of the Texas Rangers and L.A. Dodgers in recent years, the Pittsburgh Penguins have gone belly-up twice (one of many NHL teams to file for bankruptcy in recent memory), and the NBA often complains of how much money it loses. Still, domestic teams sell for hundreds of millions, if not billions, of dollars.

It would make sense then that the public markets would be dying to get a piece of such a well-known franchise as Manchester United -- or so the Glazers thought. They originally priced the IPO at $16 to $20, which would have put a value of up to $3.3 billion on the club.

Instead of gobbling up shares, investors scoffed at that price and the company went public at $14 with a $2.3 billion market cap. That's still expensive for a company that made $20.4 million from continuing operations last year, but it isn't as insane as what the Glazers originally thought they could get.

Investors seem to be focusing on bottom-line profits these days, a refreshing change from the bubbles of the Internet and housing boom. And it isn't just new IPOs where sanity appears to be winning out.

Rationality in other markets
The price of oil has bobbed around $100 per barrel for over two years now, a remarkably consistent price for a commodity that broke $140 per barrel in the lead-up to the Great Recession. Has the energy market settled down from its volatile run in the 2000s? Prices have spiked and dropped periodically as Libya, Syria, Iran, and even Europe throw challenges the market's way, but overall the commodity has reacted very sanely to economic and political news, and in the long term has been driven more by supply and demand.

The same could be said for natural gas, where supply and demand has driven the price since fracking caused a boom in production. The price has plummeted as fracking expanded, forcing drillers to cut back on natural gas drilling, a very sane reaction to supply and demand changes.

Sanity returns... for now
The market's reaction to IPOs from Facebook, Zynga, Pandora, Groupon, and now Manchester United has me thinking that maybe, just maybe, we've learned some lessons from the boom-and-bust days of hot investments over the past 20 years. Sure, there'll be another bubble, there always is, but for now earnings and growth appear to be driving the market -- just the way it should be.

Just because the market has reacted negatively to IPOs like Facebook doesn't mean these companies are bad, just overvalued. If you want to know when Facebook may actually be worth buying, check out our detailed report on the stock. It comes with a year of free updates on the stock, including our expert's take on earnings each quarter. Find out more here. You can also check out our latest on Zynga.