The user data helps/ed Google create the superior UX, as you say. The reach is what makes Google & FB valuable to advertisers. A search engine with 0.1% of Google's user volume cannot charge advertisers 0.1% of Google's as revenue. Returns to scale/reach/market-share are very substantial in online advertising.
I'm glad we're talking though. Those tech giants are too powerful.
Ultimately, the old antitrust toolkit is near useless today, for dealing with tech monopolies. It's not obvious what "break up Google" even means. There are strong network effects and other returns-to-scale. It's a zero-marginal cost business, which was rare enough in the past that economists a ignored it.
We need fresh thinking, a new vocabulary, new tools, but we do need to deal with it.