There seems to be quite a bit of confusion on fanfiction.net about song lyrics and public domain. The confusion is deep-seated enough that some authors are resisting when reviewers point out that they have used song lyrics illegally, since the authors reason that they own the cd. Owning the cd does not confer any rights to the song or the lyrics to the purchaser. Consider; radio stations must pay royalties to the artist whenever the station plays the artist's song. That fact, in and of itself, shows that the song is not public domain.
Public domain songs are typically ones that are so old, and well known, that no one is gaining royalties from them any longer. Examples would be "Mary Had a Little Lamb" or "Rudolph the Red-nosed Reindeer" (and even those I'm not entirely sure of!)
Contemporary songs that are currently being played on the radio, or still in primary release (such as cds that you can still buy in a store) are most likely NOT going to be public domain, unless the artist has said that the song is.
( Wikipedia link and select article quotes about Public Domain )Simply citing the original artist is not sufficient. In order to have the ability to post non-public domain lyrics, the author would need some sort of documented proof from the person who does own the copyright that he (or she) has permission to post them. (btw - this, too, can be done. I haven't attempted it with song lyrics, but I did formally request, and receive limited use license (basically, permission) to use Transformers images for bowling team shirts, from Hasbro!)
I'm not trying to blast anyone at all, this is intended to perhaps help clarify things, or at least point some writers in the right direction to research the matter for themselves. I'd hate to see decent stories pulled from ff.net because authors mistakenly think that since they own the cd, they have the right to post lyrics in their stories.