Also a lot of C level execs work on different vesting schedule and get allocated shares as bonuses. The whole point being this question is largely moot on a lot of levels any stock left on the table is probably a very small portion of what he was bound to make on the IPO via a standard option vesting.
There was also a clue in their S-1 [1] where his name is not included in the 'executive compensation' section, generally that is a sign that either he won't be continuing on with the company going forward (likely case) or that if he quit it would not materially affect the company (hence not of interest in the prospectus)
Given that he's been there 3 years out of four, has a C level position, and all of the other C level jobs have multiple millions of shares, lets give him a 'lowball' grant of 4M shares. So 3/4 are vested and that is 3M shares. So FB is currently $30/share but lets say he sells all 3M and pays 50% in taxes. Lets be even more pessimistic and say that his sale pushes the price down to $25, that is 1.5M shares @ $25 that he pockets free and clear. So $37.5M in the bank. Perhaps he did this math and said, "You know, I've got lots of things I want to do and that chunk of money would give me the freedom to do it until I die."
[1] http://www.sec.gov/Archives/edgar/data/1326801/0001193125120...
He probably has restricted stock units and common stock with a buyback provision. As far as I know, Facebook stopped granting options before Taylor joined Facebook.