Which is odd, considering they should be on by default. So unless someone manually removed them (Admincp - User - Identity Services), and to the best of my knowledge, they cannot be turned off.
The fact there is a header "Identities" makes me think that they indeed did get deleted.
Edit: To re-add Twitter at least, use the following values in the fields:
Service id: Twitter
PHP Model Class name: XenForo_Model_IdentityService_Twitter
Service name: Twitter
Account name description: @username