because it's very well designed. it gets casual asynchronous messaging right in a lot of ways. unlike email, threads are grouped by person rather than by subjec

because it's very well designed. it gets casual asynchronous messaging right in a lot of ways. unlike email, threads are grouped by person rather than by subject line, which makes it a lot lower friction (subject lines are for talking about something; they're a usability drawback when you're primarily talking to someone). unlike sms, you don't have to maintain a person->id mapping yourself, it's done globally and kept constantly up to date by facebook. unlike whatsapp, it's not tied to your phone; you just need to know your username and password to access it from any browser-capable device. and finally, it supports notifications via a variety of mechanisms, and even if you don't enable any of them (i don't), most people who use facebook check it regularly anyway, so you get to see the little "you have new messages" icon each time you go to read your stream.