Facebook etc, while politically biased, keep profit as a target above their views, for the most part.
Brexit? I remember most news and ads were selling "no" yet "yes" happened. I recall the "no" voters either were of the category that wanted to vote against the political party in power, or of the type of voters having had enough to be told how to think.
As for the further turmoils you might be correct though. The lockdown ads campaign didn't go without some opposition but got sold to a large enough audience.
I think you are confusing being opinionated "tinted" of sorts, as newspapers are, with microtargeting. a progressive newspaper will have progressively tinted reporting, and you get what you buy. and the newspaper is showing every reader the same content for the same article.
microtargeting means you identify personal traits and you present a content that fits your profile. probably pro brexit? contents that engaged you to go and vote and contents that you will more likely share with like minded people. probably against brexit? deter you from voting, by giving you a false sense of the outcome, or by frustrating you just enough. and so on.
I was shocked when I realized that political campaigns create dozens of variations of contents tailored for different combinations and varuations of personality parameters. the wasp lesbian gets another one their than the Catholic than than the black atheist.
and in this game Facebook keeps their own opinion out. they just offer means for the microtargeting, so the different messaging reaches their individual targets.
and that is the problem I have with it.