There is connection for facebook and twitter, but unfortunatley not for google plus.
It is a pretty easy functionality to add.
Seriously though, it's probably just that there are mány more people using facebook and twitter than Google+ and the admins had to make choices so not to make Duolingo into yet anóther social medium.
They had to make choices? Why is that?
I don't know how to connect my Twitter to Duolingo :(