Facebook is popular for these things but that’s because Facebook had a big user base, not because they keep competitors from forming.
They have a network effect that smaller competitors don’t. Thus, at the end of the day it’s the user’s choices that keep Facebook a sort of monopoly in those areas.
Yeah, I'd say from 2004 - 2015 was the heyday for me on local events for small bands, house shows, and punk/DIY venues. Eventually FB Events died out socially by not being able to send invites to mass groups of friends/previous attendees, and attrition, and so on... A real shame for non-major venue events and the DIY scene.
Marketplace is semi-useful still, quasi-better than craigslist, but keeps getting filled with a lot of cruft of drop-shippers and scammers.
That's a separate legal argument and as I understand it not necessary to qualify a as monopoly.