they also picked out the key influencers (a-list bloggers) in that community and either met them face to face (e.g. fred wilson @ demo day) and/or sent them personalized, polite emails asking them to try disqus. then it trickles down from there ("if it's good enough for fred wilson's blog, it's good enough for mine."). "the tipping point" by gladwell has other examples of this.

their product is inherently viral, too ("what are those cool threaded comments on your blog? powered by disqus?")