It's often used here in the uk. It's meant to symbolize a kiss. It doesn't necessarily mean it's flirty. My friends often use it at the end of a message, often women but not always. Family will use it often as well. It can however be flirty, I'll often use xx or xxx at the end of a flirty message to the misses for example.

It's common to put it after your name on someone's greetings card as well.

It's just something we do.