So, I'd guess, if there's was least 7k public servers in 2008, it won't be wrong to say "thousands."
http://xmpp.org/xsf/press/2003-09-22.shtml - this one is old, much before the popularity peak, when XMPP was still called Jabber, and yet: "the JSF's estimate includes more than 4 million paying customers of Jabber, Inc.'s commercial software as well as an estimated 6 million users of open source and other commercial implementations of the Jabber/XMPP protocol"
So, I guess, "millions" isn't a wrong estimate, too.
http://www.process-one.net/en/imtrends/article/usage_estimat... - "that study relies on a panel of 7292 XMPP domains as discovered by the IMtrends engine"
So, I'd guess, if there's was least 7k public servers in 2008, it won't be wrong to say "thousands."
http://xmpp.org/xsf/press/2003-09-22.shtml - this one is old, much before the popularity peak, when XMPP was still called Jabber, and yet: "the JSF's estimate includes more than 4 million paying customers of Jabber, Inc.'s commercial software as well as an estimated 6 million users of open source and other commercial implementations of the Jabber/XMPP protocol"
So, I guess, "millions" isn't a wrong estimate, too.