Twitter recently changed their Terms of Service (i.e. TOS). They (somewhat) address the changes in a blog post, that generally outline each change, most notable their new found ability to advertise and their redefinition of ownership:
Ownership—Twitter is allowed to “use, copy, reproduce, process, adapt, modify, publish, transmit, display and distribute” your tweets because that’s what we do. However, they are your tweets and they belong to you.
Essentially, while a copy of your tweets may still “belong to you,” Twitter now claims ownership over a copy too and they are reserving the right to do whatever they want with it. So, how exactly do my tweets still belong to me, if Twitter now owns them?