> That’s like saying you ordered a cheese pizza for $10, the company unilaterally charges you for $1,000 and justifies that because they bring you 100 pizzas...at no point was there consideration for the modification of the agreement even though you got 100 pizzas. Similarly can FB just start collecting more data/selling to third parties by unilaterally changing their terms and justify it by saying a unilateral change in service equals legal consideration?

I don't think this is quite a fair comparison. Let me rephrase it and tell me if you still think its illegal (unethical, sure, but illegal):

You subscribe to a pizza delivery service that sends you 1 pizza a month for $10/month. After a few years, they decide to up the minimum subscription to 100 pizzas a month for $1000 (or maybe even $500, there's probably some reasonable economies of scale arguments to be made about FB). They send an email notifying all customers of the ensuing upgrade, and a few months later you start getting charged 50x more and getting a whole lot more pizza than you want.

That's still probably not a good thing to do, and I wouldn't fault you for calling FB shady if they did that. But I'm also not at all certain that that action is illegal, especially from a strictly contract law perspective.