Facebook is blocking ad blockers on the desktop version of its service, saying well-made, relevant ads can be "useful." At the same time, the world's biggest social media company says it is giving users easier ways to decide what types of ads they want to see - unless, of course, the answer is "none."

Ad blockers filter out ads by refusing to display page images and other elements that originated with a known ad server. But Facebook has found a way around this. Beginning Tuesday, the desktop version of Facebook will show users ads even if they have ad blockers installed. The changes don't affect the mobile Facebook app, which brings in the bulk of the company's advertising revenue. As with most new Facebook features, the changes are being rolled out to users over time, so some people might see it before others.

While couching its move in the language of customer service - primarily by reiterating its premise that ads serve a purpose if they're relevant and well-targeted - Facebook is also upfront about needing them to make money. Andrew Bosworth, a Facebook vice president, pointed out in a blog post that Facebook is a free service that's only able to operate because it makes money from advertising.

In the most recent quarter , Facebook made $6.24 billion in advertising revenue, an increase of 63 percent from a year earlier. Mobile advertising (which is not affected by the changes) accounted for 84 percent of this.

Several publishers, such as The New York Times have tried to work around ad blockers by asking users with ad blockers installed to turn them off in order to be allowed on a website. Other technology can "reinsert" ads that have been blocked. But there are ways to configure ad blockers to stymie these efforts as well. Facebook's ad-blocker blocker works by making it difficult for software to distinguish advertisements from other material published on Facebook, such as photos or status updates. But while users won't be able to stop ads from showing up, Facebook says it wants to make it easier for people to control the types of ads they want to see. For example, if you don't want to see ads from a specific business, or ads that target a specific category like travel, cat owners or wine lovers, you can say so.

"We also heard that people want to be able to stop seeing ads from businesses or organizations who have added them to their customer lists, and so we are adding tools that allow people to do this," Bosworth wrote.

Meanwhile, Facebook rejected claims made by Germany's state authorities that it was reluctant to co-operate with them on criminal investigations, saying many of the requests it received for user data were incorrectly formulated. Several regional interior ministers have complained that the social media group is hesitant to respond to requests for data and have called on the Federal Justice Ministry to introduce new laws. But Facebook said it had provided "round the clock assistance" to the authorities in Bavaria following a spate of violent attacks in Munich, Wuerzburg and Ansbach last month. A spokeswoman for the Justice Ministry said it was examining whether there was a need to change the law or whether there was a problem with its implementation. A recent spate of attacks in Germany has highlighted the importance security agencies give to working with social networks to uncover possible links to terrorist groups.

Police said the Ansbach bomber had six Facebook accounts including one held under a false identity. Traces of an online messaging conversation found on his phone also suggest he was influenced by an unknown person up until the time of the attack, Bavaria's interior minister said. Germany's spy chief called on Monday for a more intensive exchange of information between social networks and security agencies in the fight against terrorism.