Yes, there was a guild purge in <The Zerg> of about ~300 toons that were inactive for quite some time.
SELECT*
FROM dbo.thezerg20120121
WHERE LastOnline >'0 years 04 months 00 days 00 hours'

-- 272 Rows Returned

If you install the InterGuildLink addon you can see the members from the other guilds that have the addon insalled. You can also type /who The Protoss or /who The Terran to see a list of the members that are online in those guilds.

There is no specific purpose for the other guilds.

There never really has been much chatter that goes on in gchat. You'll typically find more activity in vent and anyone is welcome to join in on the fun.