Social networking Web sites have become hugely popular, but whether such businesses can generate big money is still unknown.

MySpace.com, Facebook, Bebo and other sites have been earmarked as key locales to place ads because of the growing millions of people who use the sites to share messages, swap photos and otherwise be "social" with other users.

But Google (GOOG) and others are finding that millions of users don't mean much if those users ignore the ads, says Debra Aho Williamson, an analyst at eMarketer.