Google Talk support for XMPP: 2005-2013
Facebook Messenger support for XMPP: 2010-2015
Jabber.org support for new accounts: 1999-2013
First-class integration with two of the world's largest social networks put XMPP in practically everyone's hands for a time, but when all the major hosts left, network discoverability and typical account longevity dropped drastically. The landscape is bleak today.
And since then, our collective needs and expectations of a chat platform have expanded. XEPs have been developed to bolt much of that functionality onto the base protocol, but that has led to a fragmentation problem on top of the bleak server landscape.
This unfortunate situation might be navigable by a typical HN user, and perhaps we could guide a few friends and family members and promise to keep a server running for them, but I think the chances of most people succeeding with it are pretty slim today.
Facebook never had "first-class integration". It was just a client bridge - you could login into Facebook Chat using your XMPP client, but it was a completely separate network, unlike Google Talk which was an actual federating XMPP server.
Fair enough. (Although all the XMPP clients that I used supported multiple accounts, so it made little difference from where I was standing.)
In any case, it contributed significantly to XMPP's reach and utility, and it's gone now.
(And my point regarding support on Google/Facebook was that their users could chat with me over XMPP without having to leave their familiar sites, sign up for anything new, or do anything else special. That put it in easy reach of the masses.)
The same could be said about various XMPP transports that I've used back in the day with Google Talk to access all sorts of IM networks. Facebook was just running one on their servers rather than you running it on yours.
Ultimately they just briefly used XMPP to not have to implement their own desktop client for their closed proprietary network. It had nothing to do with network reach, unlike Google Talk which did actually bring XMPP to the masses for a while (and then took it away).