It was a stellar debut day for LinkedIn. After hitting a day-time high of $115, LinkedIn shares ended the day at $94.25, more than double its IPO price of $45. 

At the closing price, LInkedIn commands a market cap of $8.91 billion. Its shares are a far cry from the $17 price it traded at on the SecondMarket trading platform one year ago. 

It’s little wonder why: LinkedIn is the first major U.S.-based social network to go public, and the ballooning demand for LinkedIn stock is spelling out huge possibilities for the long-dreamed-of IPOs of Facebook, Twitter, and Groupon.

If the report from SecondMarket is any indication, those IPOs are sure to explode with a fiery vengeance. Of the top 10 most watched companies on SecondMarket, social media dominates the top five—starting with (you guessed it) Facebook. Twitter comes in at number two, while Groupon comes in third, LinkedIn fourth, and Zynga fifth.

As of March 2011, LinkedIn had 102 million members, which was 12 million more than it had just three months earlier in December 2010. In year-over-year metrics, the base of registered members grew at a compounded annual growth rate of 76% between 2008 and 2010.

In addition to its growing member base, LinkedIn also reported growing revenues. In 2010, the company net revenue more than doubled to $243.1 million from $120.1 million in 2009. Even more surprising, net income rose sharply to $15.4 million from -$3.9 million in 2009.

All of this leads to some mouth-watering fantasies for the day Facebook goes public. With some 600 million members, Facebook has a user base six times larger than LinkedIn's--not to mention a valuation some 7.5 times higher than LinkedIn's current $8 billion. 

And then there's Twitter, which has some 300 million registered users--which is triple that of LinkedIn--with some 460,000 new accounts being added each week. And of course, there was Twitter's recent speculated (and highly controversial) valuation of some $7-$10 billion (even though its yearly revenue is only $100 million--fifty times less than a $7 billion valuation). 

Or one can look at Groupon, which is rumored to be considering an IPO this year at a $20 billion valuation. As of March 2011, Groupon had some 70 million subscribers, which is obviously 32 million less than LinkedIn, but Groupon's growth has been a force to reckon with. The company is no longer sharing its metrics, but my guess is that Groupon is up to 90 million subscribers today. 

So what does all of this mean? Of course there's the ominous "tech bubble" talk, which LinkedIn CEO Jeff Weiner brushed off when a CNBC reporter brought it up this morning. "At the end of the day, it's not about the share price -- it's about our continuing to execute," he said.

Earlier this week, LinkedIn surprised many by raising its IPO share price from between $32 and $35 to between $42 and $45. But the outlook appeared somewhat dim given the recent flop of Renren (RENN), which debuted on Wednesday, May 4, at $19.50, and by the end of the day, it was down $18.01. As of 10:48 EDT today—just two weeks after debuting on the market—Renren is down to $13.92. Although it’s worth noting that that is a slight uptick from its share price yesterday, which closed at $12.84.