This might be the case if not all files were deployed onto your server properly. Did you follow the installation manual? http://wiki.plumrocket.com/wiki/Magento_Twitter_and_Facebook_Login_v1.x_Installation
Optionally, you can try to:
1. Remove the Magento 2 Twitter&Facebook extension using this manual:http://wiki.plumrocket.com/wiki/Magento_2_Twitter_and_Facebook_Login_v2.x_Uninstalling
2. Install the extension again.
Hope this information helps. As a side note, you can submit a Help Desk ticket if you fail with the above steps:https://store.plumrocket.com/contacts/
This way our specialists can briefly check the issue for you. But first please make sure to do the above steps, this should resolve the issue.