I am not a lawyer, but I do know that HR departments at larger companies coach their employees not to speak poorly of any ex-employee for fear of defamation lawsuits. The industry standard even for horribly incompetent employees is to say only when the employee worked for you, what their salary is, and on the very edge of acceptability is "would you hire this employee again?" as a simple yes/no only question.

Most employers would have a fit if you got on the phone, even in a private conversation, and bad-mouthed an ex-employee you managed because of defamation lawsuit fears. And that's from a one-on-one, he said/she said deal. Blasting this stuff over twitter is so much worse.

tl; dr -- Zynga's lawyers are probably in crisis mode right now.