It's likely that they were already in the process of closing the round (i.e. had signed some term sheets) prior to having the acquisition conversation with face

It's likely that they were already in the process of closing the round (i.e. had signed some term sheets) prior to having the acquisition conversation with facebook. At that point they probably were legally bound to close the round prior to the acquisition (and is likely a reason the company went for such a high price... the investors wanted to at least double their money).