As Microsoft Corp. makes a $44.6 billion bet on Internet advertising with its unsolicited offer for Yahoo Inc., there are signs that some of the biggest new places where consumers are flocking on the Web — social networking and video-sharing sites — are yielding advertising revenue slower than some Internet companies had hoped, the Wall Street Journal reports.

The latest warning that the hottest Web properties are proving difficult to make money from came from Internet giant Google Inc. While announcing disappointing fourth-quarter earnings Thursday, Google executives said the company was having a harder time than it expected generating ad revenue on social-networking sites and figuring out the best ad formats for its YouTube video-sharing service. Social-networking phenomenon Facebook Inc. also has been publicly grappling with how to make money amid its massive spurt in usage. Microsoft, which owns a 1.6% stake in Facebook, has a long-term deal to sell ads that appear on the site — and analysts estimate that arrangement is losing money for Microsoft.

The challenges of making money from social networking and user-submitted videos are potentially significant for Microsoft as it pursues Yahoo. A central focus of Microsoft’s efforts is to access Yahoo’s 500 million-strong global user base and combine the online ad systems of the two companies.

The slower-going also adds uncertainty at a moment when many are anxious that a broader consumer slowdown could crimp online ad growth. While Yahoo last week said 2008 was starting out solidly for the company, it saw weaker online ad spending in the fourth quarter among some categories of advertisers — such as finance, travel and retail — affected by broader economic issues. Since advertising on social-networking and video-sharing sites is still largely experimental for marketers, it could be more vulnerable in an ad-spending pullback.

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