Google and Facebook are both often used as examples exactly because they were "late" to the party. That seemed like a bad idea, but they both had a secret (as Peter Thiel likes to say), that made it not a bad idea.
I don't recall anyone considering Facebook a bad idea either, just that it was "yet another social media" site. Facebook mostly lucked out because of MySpace's user exodus.
Facebook is a bit different in that the market they went after was the right one not that their product was that much better. Zuck may have been lucky at the start, but he soon realised that he was on the right path and stuck to it.
First-mover advantage is overrated, and in the tech industry, you'll notice that many if not most successful companies were not first-movers. Often, first-mover advantage is a disadvantage.
In even the most established markets, you can find meaningful niches and adjacencies that the big players aren't tapping. Many times, the untapped adjacencies can expand the overall size of the market significantly (see Airbnb).