Yes. But it's not happening with fixed line ISPs as i know. It only happens with mobile data usage in EDGE and 3G. Mostly facebook.com and twitter.com can be used unlimited for free or for a very low cost add-on plan. Other sites accessed are charged as per the normal plan.

Even though it appears sinister, the mobile operators do this to attract young customers who use these sites the most on the move. Personally, i don't agree with this setup.