But not the people. Empty forums are boring. They have the mindshare. There were alternatives to Twitter (some of them open source) but no one has fled to them no matter how much they cripple the APIs or how many ads they put.
Remember MySpace? They were the new hotness until they imploded. Reddit could easily turn into yet another ad-festooned wasteland, and many people are tired of the "piss off your users as much as you can get away with" business model.