How the Portland Rally Grew So Big

the organizers didn't plan for such a large event, but word got out--fast:

nytimes: The event‘s elated organizers say, however, that they did not deliberately set out to shatter records. It was only on Wednesday night, less than 100 hours before the start of the rally, that they decided to risk holding it outdoors, and after that choice was made, they had trust that word would spread quick enough, through media traditional and new, to justify their decision to eschew a smaller indoor arena.
“People in Portland are very wired, in both the Internet sense and otherwise,” Nick Shapiro, the Oregon communications director for the Obama campaign, said in an interview after the rally. “On Thursday, we sent e-mails to key supporters who we knew would spread the news, but we also relied on television and the newspapers and radio to get word out.” Flyers were also posted at restaurants and stores around town.
Mr. Obama was making his fourth appearance in the state, but his first at a large venue in Portland since a rally in March at the local coliseum, where Gov. Bill Richardson of New Mexico endorsed him. His campaign had announced Saturday that he would watch the primary results come in Tuesday night in Iowa, not Oregon, so for both the faithful and the merely curious, it was their last opportunity to see and hear the candidate before the vote.