Each network provides different features and abilities. We think that it's possible to add auto-login functionality for Google+ in the future, and it's something we're investigating. LinkedIn and Twitter do not support any sort of auto-login ability from the investigation we performed a few months ago.. and I don't expect that to change soon.
G+ auto-login (as well as a few others that support it) should be coming over the next few releases though, if all goes well.
I hope that helps answer your question. If you have any other feedback or suggestions, just let me know.
Thanks,
Alex
auto login
