The difference is at that point Mark had something worth that much money and had already spent several years building it. If he decided in 1 year he was done he could have probably found another buyer at a valuation high enough to retire unless he royally screwed up the company.

Turning down $100mm when you have nothing of value yet is very different imo

Yes, but they turned it down to work on what they liked. This is just paying you upfront to do that. Granted you don't own it...

I'm not positive, but I think Spiegel already made FU money as part of a funding round prior to the FB offer.

Can anyone confirm?