i don't know about others, but i personally love the building phase of any project - you research, tinker, and really focus on the problem as a whole, but after that i get bored very quickly. Maintenance/bug fixing, adding features, upgrading, and even expanding to new domains always feels like a grind, i may get a similar high when new things 'work' but no where near the same potency. I'm still early in my career phase (late 20s), but i've already taken large pay cuts (several times!) just to build again.
I'm sure Facebook has a lot to offer in terms of scale, but Uber probably has a lot to build. If Mr. Sullivan is anything like i am, it was an easy choice (not necessarily about the money).