When I interviewed at Facebook almost every person I talked to said "if Google blocks ads by default Facebook will sue them" so I expect to see a lawsuit eventually
Facebook seem to have done an incredibly good job of reducing their exposure to the browser. That comes at the cost of exposure to Android, but it feels like Google are going to find blocking Facebook ads in the Facebook app way harder than in their website.
That's all a little bit irrelevant though because Facebook are members of the 'coalition for better ads' anyway.