For some time, Procter & Gamble, the world’s largest advertiser, has been dipping its big toes into the vast pool of Facebook, now the world’s largest social network. I recently knocked on the doors of both companies to hear how the experiment was going. Neither was inclined to say much.

Independent experts on Web advertising have been watching, however, and what they see is a myriad of difficulties in making brand advertising work on social networking sites. Members of social networks want to spend time with friends, not brands.

When major brands place banner advertisements on the side of a member’s home page, they pay inexpensive prices, but the ads receive little attention. Seth Goldstein, co-founder of SocialMedia Networks, an online advertising company, wrote on his Facebook blog that a banner ad “is universally disregarded as irrelevant if it’s not ignored entirely.”

When advertisers invite members to come to pages dedicated to their products, they can attract visitors only by investing in expensive creative material or old-fashioned promotions like prize contests.

And when they try to take advantage of new “social advertising,” extending their commercial message to a member’s friends, their ads will be noticed, all right, but not necessarily favorably. Members are understandably reluctant to become shills. IDC, the technology research firm, published a study last month that reported that just 3 percent of Internet users in the United States would willingly let publishers use their friends for advertising. The report described social advertising as “stillborn.”

All Web sites that rely on ads struggle to a greater or lesser extent to convert traffic, even high traffic, into meaningful revenue. Ads that run on Google and other search engines are a profitable exception because their visitors are often in a buying mood. Other kinds of sites, however, can’t deliver similar visitors to advertisers. Google’s own YouTube, which relies heavily, like Facebook, on user-generated content, remains a costly experiment in the high-traffic, low-revenue ad business.

Financial data would show the current state of Facebook’s advertising, but none are available. Facebook is privately held and a spokesman told me that it does not disclose revenue or any information about its ad sales.