It's pretty simple math.
About 30% of that 750 million is already gone to legal fees and paying off the original people that sued.
That leaves you about 525 million.
Facebook had, on average, around 2 billion users during that period.
If only 10% of them sign up, that's 200 million people.
That comes to $2.63 if it's split evenly, which it isn't. The more time you were there, the more you get. I was there all but one year of it.
So even in a best case scenario, it's not going to be more than 10 bucks. It would have to be pretty much nobody signing up to get more than that.
20 bucks is the best case scenario for folks that were there the entire time.