​Search advertising is a $15 billion market. Facebook, currently planning a $5 billion IPO which would put the company’s market value at around $100 billion, wouldn’t be muscling into the search business unless it thought it could capture a significant part of that $15 billion market. And where would Facebook’s share come from? Mostly from Google.

​Alas, poor Google. Dominating a technology market on Monday guarantees nothing about dominating that market on . . . Wednesday. And as for Friday, TGIF is strictly n/a in the high-speed technology business because developers, unlike government regulators, don’t take weekends off.

In the high-speed technology business Nokia and BlackBerry were the dominant handsets on Monday — until Wednesday when the iPhone came along. Sirius and XM discovered that the real competition to satellite radio was streaming music, the iPod, and old-fashioned radio. In 2010, regulators were thinking about stopping Google’s purchase of the advertising company AdMob — until Apple acquired Quattro Wireless, a competitor of AdMob, and became a formidable powerhouse in mobile ads. So the FTC approved the Google/AdMob deal. On that same theory, the FTC and other antitrust authorities should now conclude that Google faces a competitive market in search.

​Of course, Facebook isn’t Google’s only problem. Microsoft, the world’s largest software company, is also aggressively competing, using its Bing search engine. And Twitter, too, threatens, having revised its search function and added advertising to its search results.

​According to Bloomberg BusinessWeek, Facebook’s entry into the search business is being led by a former Google executive. Because Facebook already possesses so much user information — it knows more about Facebook users than Google knows about Google users — Facebook is in a strong position to develop individually tailored search results quickly, giving it an immediate advantage over Google. That poses a serious competitive threat to Google in search advertising.

​Facebook’s business plan isn’t public, of course. It may be attempting to capture search advertising based on friends’ recommendations because, according to its executives, those ads are three times as valuable as ordinary ads — and as a public company, Facebook will have new pressures to produce returns for shareholders.

​Or Facebook may only be attempting to keep users from leaving their site, given that search is such an important part of the Internet experience. Facebook users already spend more time on the Facebook site than they do on Google, Yahoo, MSN, and AOL sites combined.Adding a search component will increase that time. Wall Street analysts applaud the move. As one said, “Facebook obviously owns social networking but needs to rapidly grow through generic search and mobile.”

​Google, countering the new search threat from Facebook, has recently made its search more “social.” One analyst described Google and Facebook as “scorpions in a bottle.” Nowthat’s competition. Who wins? Consumers win.

​Major Internet companies are crowding against each other like drivers in a NASCAR race, and old, narrow, market definitions simply do not reflect the dynamics of the modern market. All this jockeying for position is good news for Internet users: more competition will improve all search engines.

​This competition should also be instructive for the Federal Trade Commission, the several state Attorneys General, and the European Commission who are investigating Google for its alleged dominance of search that they should show actual harm to consumers before seeking to interfere with how markets evolve. This competition should also demonstrate to them that search is a well-working, highly competitive market where change happens rapidly — the sort of market regulators should let flourish on its own.

​For regulators to prove that a monopoly exists, the supposed monopolist must have both(a) high market share and (b) a durable market position. The efforts of Facebook, Microsoft, Twitter, and other companies deny the second condition and threaten the first. What looks like Google’s “dominance” on Monday may be just a memory by Friday — a memory of dominance dethroned by the whims of users, hundreds of millions of users, able to switch preferences with a single click.

​And another click. And a click-click-click. A click-click- creaking, click. A cascade of hundreds of millions of cracking, croaking, crumbling clicks that collapse any serious antitrust case against Google today or other technology companies tomorrow.

Daniel Oliver was chairman of the FTC from April 1986 to August 1989. He is an adviser to Google but his thoughts and views are his own.