Facebook might be one of the most uptight sites on the internet, requiring everyone to log in with their name and picture and birthday and to generally behave themselves like a nerd in study hall. Yet it still has a slimy underground where people do slimy things, the company conceded Friday. Facebook says it is removing fraudulently generated “likes,” which helped make some businesses on Facebook appear more popular than they really were.

In a post on its security blog, Facebook wrote that it “recently increased our automated efforts to remove [fake] Likes” on business pages. In other words, business pages were previously puffed up thanks to shady clicks from fake or hijacked Facebook accounts.

Facebook is dumping this news at a time when few people will read it, on a Friday before a holiday weekend. And no wonder: The implications are disturbing. Facebook’s advertising system is built on the idea that consumers will be willing to build closer relationships with advertisers, “liking” the advertiser’s pages, reading the advertiser’s status updates, and circulating content about the advertiser to friends. If an advertiser's popularity is exaggerated by fake “likes,” it makes the business less trustworthy and less likely to be engaged by real consumers.

Similarly, legitimate advertisers need to be able to trust that Facebook is populated with real people engaging in real activities. If the social network becomes a haven of bots and fake clicks, no business will want to buy advertising, often billed on a per-click basis. At least one business has already claimed its Facebook advertising spend was squandered on bot clicks.

Facebook isn't saying outright that businesses paid for fake clicks. But it is implying as much in today's post, referring to “vendors” of fake likes and adding, “we do not and have never permitted the purchase or sale of Facebook Likes.” And there are signs of a veritable underground "like" economy. The ad group Conversation Agency, for example, told Business Insider its clients are losing large numbers of likes due to the crackdown and are in an uproar. (Facebook will only say that the typical Facebook page will only lose about 1 percent of its likes due to the crackdown, a fairly meaningless statistic because "likes" are not distributed evenly among pages; more popular pages could be seeing a much more pronounced decline.)

Facebook says the crackdown “will be a positive change for anyone using Facebook.” But that’s not true. Fraudsters are clearly using Facebook, too, hence all the fake “likes.” And they’ll be racing to thwart Facebook’s filters. Summer ends this weekend with a victory for Facebook’s "like" engineers. But the arms race has just begun.