It's how they talk ironically on the internet, not in real life, although I don't know if that distinction is as clear cut for young kids as it is for twenty/thirtysomethings.

Fam is/was used in london as part of the normal youth vernacular but it's leaked into general internet speak via barbershop-based twitter memes: http://i2.kym-cdn.com/photos/images/original/000/920/788/68e...

They've got the grammar wrong however: one cannot be "a fam."

While you could say to someone, "pass me that glass bro" but also say "all my bros were there," only the former usage is valid with "fam." Although you could probably get away with it if you pronounced it "famz" and replaced "were" with "was". London is funny.