What every end-user needs to know about "public" data and privacy: Regardless your "privacy" settings, you shouldn't put something on the Internet that you would be ashamed to be made public, either now or in the foreseeable future.
He makes an interesting point though about intuitive, human expectations for data retention. In interactions with humans, we can expect passing comments to eventually pass, but a computer could retain even the most trivial of information indeterminately.
It's probably easier to educate developers than users, because there are fewer of them. Also, developers have more to lose in lawsuits and bad press, while a user may only have their life ruined.