this could be overkill for you but after turning 18 i decided to use the law to my advantage whenever possible and to try and avoid getting booked for shit. you uttered a threat with the offer a compensation for it to be carried out. thats not legal... you could depending on how things spun find yourself in a world of shit.
but as for facebook your girlfriend is being liabelously slandered in a public forum, bringing disrepute to her in her community. (this is canadian law i'm unsure if it applies to USA) you could bring forth a charge in that manner, even have a cease and decist issue ordered.
or you could go with the easiest and stop talking about it period, people doing that shit want attention, just like you did when you offered to pay for her teeth to get knocked out. if you stop giving them the attention and ignore it, it goes away. did any of this REALLY effect you?