Don't take responsibility for content people create on your site, or you open yourself up to legal liability for it.<p>Most services like posterous, twitter, Wordpress.com etc. prohibit users posting defamatory or harassing material, giving you the right to enforce it but not the responsibility.<p>As for timelines that are simply controversial rather than harassing, you really don't want to become the speech police. As you've already said: your app is just a medium. It's not your speech, you don't have to agree with it. If it's legal, you should allow it. Users hate arbitrary restrictions (LiveJournal has spent sooooo much time trying to do this).<p>As always, for the actual legal implications, you should consult a lawyer. I'm not one, and my opinions aren't legal advice. =)