I’ve been using XMPP for quite a while, first on jabber.ccc.de, then on my own server (and on a Google account in parallel), but have so far not received a single spam message.
Chee Chew said that "we haven't seen significant uptake" in federation with Google Talk via server-to-server connections. The majority of the uptake Google did see was from organizations or individuals looking to bombard Google Talk users with chat spam, Chew said.
Is XMPP spamming really a thing?