The only thing that we know they store is the creation timestamp and last-seen timestamp for each account: http://arstechnica.com/tech-policy/2016/10/fbi-demands-signa...
Reading over the article again, though, it neither confirms nor denies where these timestamps are the only thing stored about an account. They explain these are the only data matching the concrete subpoena they received.
The only thing that we know they store is the creation timestamp and last-seen timestamp for each account: http://arstechnica.com/tech-policy/2016/10/fbi-demands-signa...
Reading over the article again, though, it neither confirms nor denies where these timestamps are the only thing stored about an account. They explain these are the only data matching the concrete subpoena they received.