The latest game of online merger musical chairs has stopped and it looks like Microsoft (MSFT) is the odd man out looking for a seat. Google (GOOG) is buying DoubleClick and Yahoo! (YHOO) is ponying up for the remainder of Right Media that it doesn’t already own. So now, Wall Street is speculating about which firm Microsoft, whose MSN division is a distant third in search to Google and Yahoo, will buy to make sure it doesn’t fall further behind in the online ad race.

The takeover target du jour is 24/7 Real Media (TFSM), a company that, like DoubleClick, survived the late 1990s dot-com bubble and is once again thriving thanks to robust demand for online advertising. Shares of 24/7 soared as much as 30 percent Tuesday following a report from the New York Post that suggested Microsoft might bid $1 billion, or nearly twice 24/7’s market value as of April 30,  for the company.  Shares settled down and were up 20 percent in late day trading.  Competitors aQuantive (AQNT) and ValueClick (VCLK) also shot up Tuesday.

Google is paying $3.1 billion for DoubleClick, nearly three times what current DoubleClick owners paid to take the firm private just two years ago. And Yahoo is paying what amounts to about $850 million for Right Media…even though Yahoo valued Right Media at around $200 million when it bought a 20 percent stake in Right Media for $40 million only six months ago.

These are frothy valuations to say the least. And given that shares of the three remaining publicly traded online ad firms have all skyrocketed since Google announced it was buying DoubleClick, it’s possible that the asking price might now be too high for these companies. aQuantive and ValueClick are each up 11 percent during the past month while 24/7 has surged nearly 50 percent.

Sure, one reason these three stocks have soared is that the Google and Yahoo deals validate the online ad network business. But the stocks have also climbed due to investors’ impressions that they must be the next online ad takeover target. And that may be faulty logic.

Despite all the talk about a scarcity factor in the online ad business, a quick look at some numbers from Web research firm comScore Networks shows that there are several other options for Microsoft or other big online media companies if they choose to bypass the publicly traded firms. So investors betting on an imminent deal for 24/7,  aQuantive or ValueClick could be disappointed.

comScore puts out some figures that it calls its Ad Focus rankings, which measures the total audience size of large Web sites and ad networks. The numbers reflect the percentage of Internet users that have viewed a particular site in a company’s network at least once during a given month. 

For example, Advertising.com, which is owned by Time Warner’s (TWX) AOL unit, ranked number one in March, with 86 percent of all U.S. Web users viewing a site in the Advertising.com network at least once.  (Time Warner also owns CNNMoney.com.) But in addition to ValueClick, aQuantive and 24/7, several other privately held firms also have significant reach, as measured by comScore.

Samir Patel, founder and CEO of SearchForce, a privately held software firm that lets advertisers manage keyword purchases on Google, Yahoo and MSN, said Microsoft should be losing sleep over the fact that Google and Yahoo are making deals but he agrees that the public firms are not the only ones that Microsoft could go after.  

Patel said that Tribal Fusion, which is fifth in the latest Ad Focus rankings, with 65 percent of Internet users viewing a site in its network, could be a target for Microsoft as well as VendareNetBlue, an ad network that recently changed its name to Connexus. Connexus ranks eighth and this is a company that has backing from high profile such as Idealabs, Insight Venture Partners and Oak Investment Partners. Patel mentions two other venture-backed firms, Blue Lithium and Burst Media, which also rank in the top 20 of comScore’s latest Ad Focus rankings, as possible good fits for Microsoft.

“Traffic is God,” Patel said. “And the private companies are growing fast and might be cheaper.”

So investors bidding up 24/7, aQuantive and ValueClick should be forewarned: Microsoft has other alternatives in the online ad network business.