1.A social stealer is someone who steals people. Not Kidnappers. This person could steal your friend, best friend, boyfriend/girlfriend, etc.
To get what they want. These people are usually clever and likable.
You may find it hard to pick a social stealer out in a crowd.
2. To steal the spotlight. These would be people who may lower your popularity or win your popularity. They steal friends, lovers, fans, etc. Possibly ruining someone's social life by being liked.
"She is such a fucking social stealer. My boyfriend made out with her!"
"Dude, what the hell. Everybody's calling Bobby a player. He made out with my girl. He's just a social stealer!"
"Didn't you hear? She made out with her best friend's boyfriend! Everybody's giving her the shit! I think all three of them are totally social stealing!"