Can be an ex or jealous aquaintance or colleague or just as easily someone who has only crossed your path online, on Facebook.
Their behaviour can include:
1. Unwelcome private messages until you block them.
2. Following you around from group to group to read and respond to your posts, as they can no longer message you.
3. Cloning you.
4. Creating fake accounts to try and befriend you and get access to your profile that way - failing that: to harass you.
5. Starting up hate groups about you.
A Facespook will sometimes try to pass off their abusive and harassing behaviour as "trolling" to other members of a shared Facebook group, so that the victim is left to fend for herself.
types back: "LOL I know. Did you join?"