I don't think it was thought of that Facebook would be worth $1.5T some day, but I'm positive what was on the 10th employees mind while getting paged at the middle of night was "I'm in the process of getting filthy rich". That certainly would help me focus.
They didn’t know if they would get rich.
They certainly hoped that would be the outcome. But they would have been idiots to not know most startups fail.
Facebook was a networking site for a handful of college campuses at the time. It had some great traction, sure, but I doubt "filthy rich" was a serious expectation among employees.