Innumerable Web sites have been inviting readers to rate and comment about articles and blogs for ages already. Now, Facebook has added a considerably rarer function: Users can give "thumbs up" or "thumbs down" to ads.

Judging from a particularly sophisticated example given by Facebook founder Rob Webb, Facebook members will also be able to leave feedback about why they like or dislike an ad, either by choosing a reason from a drop-down menu, or by creating a new reason not already on that list.

Webb slipped an announcement of the new ad rating system into his blog on Thursday.

"The fact that Facebook is implementing these kinds of features before they launch an Ads API shows that they are approaching mass advertising very carefully," Webb said, writing in a third person voice.

Facebook is aware that "they need users to make ads have value, and the better the ads are the more valuable their ad space will be," Webb admitted. "Also, it's quite possible that having some interaction with ads beyond just clicking them will incentivize users to click more ads."

In his own blog, Marshall Kirkpatrick of ReadWriteWeb cited a different sort of benefit to the new feature.

"Initially we thought this would be an ineffective effort, but the more we looked at terrible ads on Facebook and thought about how happy we'd be to vote them down, the more sense it made," according to Kirkpatrick, who mentioned that most of the ads on Facebook aren't brand ads, anyway. "They are personal ads, debt relief services, and other things most of us could care less about. The more we think about it, the better the opportunity to vote down such ads sounds."

Reacting to Webb's blog, one user noted that as far back as 2005, Weblogs Inc. gave users a chance to comment on ads.

Other users complained on Thursday that they weren't yet finding Facebook's new ad rating feature -- or that, if they had, the thing wasn't working for them correctly.

In his blog, Kirkpatrick credited Facebook watcher Nick O'Neill as observing that, just before launching the new ad ratings, Facebook pulled a voting feature that appeared in its Newsfeed for a little while.

Some online discussions are under way over whether a voting approach would work any better with ads. Yet during these days of increasing personalization of Web content, is a simple majority in numbers even that important?

Webb's example shows an ad for "historic lofts" in San Francisco. But a company selling this type of real estate should pay closer attention to the opinions of high tech pros in the Bay Area than to those of college students in Australia or pharmacists in Missouri, right?

If only 40 out of 500 voters lived in northern California, how much would it actually matter whether the majority of voters liked the ad? And should the majority be allowed to kick an ad off of a site, if the ad might come in handy for a relative few?

In any event, from the comments in Webb's blog, it looks to BetaNews as though Facebook might be less interested in giving members this kind of voting veto power than in (1) exploring the characteristics that work best for targeted ads on social sites; and (2) encouraging user interactivity with ads.