Twitter recommends that 330 million users change their passwords:
TWITTER chief technology officer Parag Agrawal disclosed in a blog post that the company had inadvertently recorded user passwords, in plaintext, in an internal system. Twitter has since fixed the bug, and doesn’t think any of the exposed passwords were accessed in any way. You should still change your Twitter password right now to make sure your account is secure. You may also want to setup two factor authentication.
How to secure your Twitter account on How-to Geek.