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.

He gifted his father some shares, is it that surprising he'd gift his partner/wife shares as well?

He doesn't really need one. Facebook is a premarital asset and he has no other assets. In California anything you own before marriage is yours including the gains on those assets and you can take money out and spent it on other things and as long as you can trace it back to the premarital money then your good to go.

Pre-nups address many other things as well including alimony and future earnings.