Already worried about the bubble-popping potential of a looming recession, the Web 2.0 crowd has been hit with traffic slides in recent days at three of the Web’s bellwether properties, Google, YouTube and Facebook.

Today, Nielsen released new stats from its Video Census service, reporting drops by YouTube in both unique user and total stream numbers over those in December, according to the NewTeeVee blog. Google-owned YouTube had more than 65 million fewer streams and almost 1 million fewer unique users in January, Nielsen reported.

While still growing – particularly abroad, traffic at social network Facebook has been leveling off recently in the US. Facebook unique-user visits actually dropped 2 % to 33.9 million in January, according to research firm comScore.

Perhaps the biggest hit of all came at Google, where clicks on ads in January were flat versus the year prior and actually fell 12% versus the last three months of 2007, again according to comScore.

In an industry driven by growth, flat lines over any period of time are cause for concern.

Perhaps just as interestingly, critics have called into question the methodologies used to measure Web traffic – saying inaccuracies in such reports are prevalent. The fact that such concerns were not as quick to emerge when trends were heading up rather than down, however, shouldn’t go unnoticed.

But even the measurement companies themselves have been quick to try to explain away the drops. ComScore – whose numbers were at the center of reported drop-offs at both Google and Facebook – attributed the Google ad-click slide to changes in how Google displayed its ads rather than a fall-off in business. In particular, Google has begun suppressing poor-performing ads, decreasing inventory and thus click-throughs, according to Magid Abraham, comScore president and CEO.

“Google’s revenue will not necessarily suffer from this,” Abraham said in a statement. “In fact, Google wins by providing more relevant ads for consumers and a less cluttered ad environment for marketers.”

Revenue impact will have to wait until Google’s next earnings report, but the markets clearly have not liked the news. Google stock is down in the mid-400s in recent days from a high of over 700 three months ago.