It isn’t the UI of those products that works well though! Craigslist is ugly. Facebook on the web is cluttered. Those products work because the value they provide is so strong and unique. Craiglist is useful because lots of people post stuff there. Facebook is powerful because it’s where everybody is once a day. I have no idea what the deal with Craiglist is but Facebook probably doesn’t fix the interface because it would lose too much money in the process.