Huh. http://www.theatlantic.com/technology/archive/2013/12/what-y... says

>Importantly, my defense of the third-party doctrine implies an important limit: The doctrine should apply when the third party is a recipient of information, but it should not apply when the third party is merely a conduit for information intended for someone else.

Sourced from http://www.abajournal.com/magazine/article/the_data_question...

The linked argument (which is one half of a debate), argues that the third party doctrine should apply to things that you wouldn't be able to hide without third parties. So metadata is covered, because you can't hide who you communicate with, but content is not. The data collected here is metadata (who connected to the site), so it seems they would consider it collectable legally.