My understanding is that he's been with this girl since Facebook started anyway. I'd be surprised if she did not have a fair stake in Facebook even if she were not married to Mark.
Why would she have shares? If she wasn't an employee I don't see why she would. I mean Mark might have given her some as an anniversary gift/etc at some point, but you don't get shares in a company just for dating the founder. Although I suppose she could have been part of a friends/family founding round.
It may be different in the USA, but in many countries there are legal limits on what a pre-nup can actually cover. If the relationship pre-dates the foundation of the country, it isn't unrealistic that she would have some claim on "Mark's" assets should they break up in the future.
For what it is worth, this is really very poor taste.
