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. http://creativecommons.org/licenses/by-sa/2.0/ is a good, but it's not the only licensing option available. http://www.gnu.org/philosophy/license-list.html#OtherLicenses 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 http://wiki.openstreetmap.org/wiki/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 http://www.okfn.org/ 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.