Mergers-and-acquisition activity on the Internet is casting a spotlight on one of the Net's largest forgotten online properties: About.com.The array of content sites, which owner Primedia ( PRM) is reportedly in the process of selling, is one of the most popular sites on the Internet, judging from the number of U.S. users who go to About.com at least once a month. With online advertising growing rapidly after a turn-of-the-century collapse, media companies are trying to increase their exposure to Internet advertising by acquiring online properties. Dow Jones ( DJ), for example, recently bought MarketWatch, and The Washington Post ( WPO) went after Microsoft's ( MSFT) Slate. There's no question that the advertisers are back. Bellwether Yahoo! ( YHOO) reported gross revenue of $1.1 billion for the fourth quarter, up from $664 million one year earlier. With trends like that, it's no surprise that companies reportedly kicking the tires of About.com, according to The New York Times, include New York Times ( NYT) itself, Yahoo!, Google ( GOOG), Time Warner ( TWX) and Ask Jeeves ( ASKJ). A Primedia spokesman declined to comment. About.com, launched in 1997 as The Mining Company, was envisioned as a vehicle for inexpensively building up targeted content on the Internet, against which advertising could be sold on a relatively inexpensive, decentralized basis. Subject matter of interest to Internet users was assembled and updated by a network of experts -- independent contractors who published their work under the unifying artistic direction, editorial supervision and Internet address of About.com. Nowadays, About.com comprises hundreds of subsites devoted to special interests ranging from attention deficit disorder to zoology. In October 2000, Primedia announced the purchase of About.com in a deal that echoed some of the justifications for the announcement earlier that year of the merger of Time Warner and America Online. In each case, a major motivation for the transaction was to marry the content of a traditional media company -- Primedia these days publishes roughly 120 specialty magazines -- with online distribution.