Of course, that just means you don’t see ads on Instagram/Facebook. They still collect your data, aggregate it and trade it with data brokers, so the ads you see elsewhere (not to mention prices you’re offered) will become more accurate. In fact, it’s not unlikely that the behavioural data of people who pay to opt out of being spammed with ads will be more valuable to data brokers.
Also, for those who don’t pay, the ads will get more frequent and annoying to induce them to pay. (See also: Spotify)
And if an app like Signal bypasses blocks, having it installed could become a crime.