While Libre.fm is not meant merely as a free alternative to Last.fm, we still want it to be free. As such, all the content available through Libre.fm should be freely licensed.  CC-by-sa is a good, but it's not the only licensing option available.  The FSF License list has a short list of licenses that are free and useful for non-software data.

CC licenses are not compatible with users' listening stats, because many countries have special database laws. The  Open Database License may be a good alternative. However, users should have the option to set whether their individual listening data may be commercially used. It could work like this: When a user sets his/her listening data to non-commercial, stats are either submitted anonymously to the overall database (which should be plain OpenDBL) or not at all.

The  Open Knowledge Foundation has a  Guide to Open Data Licensing. OKFN also maintains a  list of licenses conformant with its Open Knowledge Definition. Public domain-like options for data include  CCZero and the  Open Data Commons Public Domain Dedication and Licence.