Using a database of past tweets, they were able to effectively pick out the next words a user was likely to use. But they were able to do so more effectively if they simply had access to what a person’s contacts were saying on Twitter.

Source: Social media can predict what you’ll say, even if you don’t participate | Ars Technica

Services such as Facebook already build a profile of non-members. If you don’t use Facebook, but a friend who does mentions you, Facebook begins to build a profile of you based on the mention. Scary stuff, especially with what can be predicted.