That's not entirely correct. As you say, Twitter displays the original link, so it is trivial to copy and paste it from the tweet if you wish to avoid the t.co shortening. More critically though, everyone is given the same t.co link, and directed through the same path, so if you wish to block analytics and tracking locally, you stand some chance. If I correctly understand what Facebook is doing, each user is being given a customized, encrypted link, so even if you were to block all tracking locally, Facebook will still know exactly who you are, and where you came from when you followed the link.