It's not quite that simple. California, where they live, is a "community property" state[1]. This means that anything earned while married is community property - owned 50/50 by both people. On the other hand anything you had before marriage (or inherited) is "separate property" - owned 100% that person.

Due to these rules the fact they were married after - one day after! - the IPO could make a big difference if they were to ever divorce[2]. Of course, like almost every very wealthy person, they likely have a prenuptial agreement which should establish exactly who owns what.

[1] http://en.m.wikipedia.org/wiki/Community_property

[2] http://www.reuters.com/article/2012/05/21/us-facebook-prenup...