Ask HN: How to convince someone that building a Facebook killer is a bad idea?

You should probably use the same tactics that were used to prevent people from building a MySpace killer. :)

Sometimes it's better to let things be...Facebook is far from perfect and while your friends idea seems impossible he just might end up carving up a niche service...not necessarily a Facebook takeover but something that delivers stuff people will find attractive.

Instead of telling him "you can't do this", try to be positive and give him advice that may help him succeed. Have him consider a niche, angle or feature set that separates his offering from FB.

For what reasons would you want to stop him from doing this?

Maybe if his friend was going to dump his life savings into the project, I could see it being pretty reasonable to try and stop him.