Random people. People you don't know, and who seem to be there for no reason. Strangers, or barely-there acquaintances.
The ones nobody invited to the party, but who hang around anyway - no-one will ask them to leave, as no-one knows who invited them in the first place.
My Facebook friends are a mix of colleagues, school friends, and randomers I couldn't turn down in case I'd met them and forgotten about it.