The craigslist strategy is really smart. Email has way better response rates than, say, facebook, and it seems like a solid way to get past the initial "spread beyond your immediate social network" problem. That said though, it doesn't sit quite right with me. I'm aware this is mostly a problem inherent with the craislist format, but it seems like scraping tools like they used [1] are ripe for abuse, and that by some definitions, unsolicited and unrelated emails could be considered abuse. Nonetheless, it's a really smart growth strategy.
[1] http://www.craigslistscraper.net/