Here's my 2 cents worth:

If the song is already in DigiTrad, or already in another thread, then don't post the lyrics again; that just wastes space on the server. Post a link instead.

If the song is so thoroughly "pop" that you can't imagine a "folksinger" (in the loosest possible sense of that word) ever wanting to add it to his repertoire and perform it as part of a "folk" program, then post a link, if you feel like being that helpful. (You could also just suggest a method of finding it using a popular search engine. This has the advantage that it encourages pop-oriented music fans to find songs on their own and not bother us with inappropriate postings.)

For everything else -- everything that fits a very loose definition of "folk" and isn't already in a Mudcat thread or DigiTrad -- post the lyrics if you can.