Even on desktop FB's ads are unblocked (or were when I used to have a FB account). They use a significant amount of obfuscation to make it impossible to use typical adblock techniques (url filter, css selectors, simple heuristics) without breaking the rest of the site. So far as I can tell most of the filter maintainers leave Facebook's native ads alone.

I read a while back about a research project to implement adblocking with computer vision. Wonder if that might be able to do it...