"each cofounder had been hired individually – and therefore TUPE didn’t apply"
Leave it to The Zuck to find a loophole in a legal employment protection scheme.
That being said, we're talking about a co-founder here. That's not exactly the bottom rung in the employment ladder, if I'm not mistaken.
Where you're at the top, you take your chances and look out for yourself. Running to the employment benefit larder when you feel you didn't get your fair share is rather pathetic.