In the wake of "free speech zones", increased police brutality and crackdowns, and internet communication coming of age, the hard left (and hard right, too) started up online communities, which translated into real-life communities, who organize far in advance.
They start things like message boards, where like minded folks can collaborate, they run workshops, they get educated and organized. There *has* been a government crackdown on sites that "go too far", so a fair bit has been driven offline/underground.
For example, the original owner of:
http://www.raisethefist.com/Went to jail for the posting of organization and bomb-making directions on his website:
http://en.wikipedia.org/wiki/Sherman_Austin...and may you may recall the recent arrest of somebody for *tweeting* their location during a protest. We're far beyond "show up here at such-and-such a date".
edit: clean up phrasing to improve meaning