Your credentials are secure. First, Facebook requests you to authorize Convergence to access your feed and publish posts to your feed upon login. Second, Convergence does not store your Facebook password unless you specifically request it. Convergence does not post anything to your feed that you did not post. Finally, Convergence does not share your information with other Convergence users who are not your Facebook friends.
Note - At no point does Convergence store your Facebook user name or password.