I am sure that many people learn their songs from YouTube and it is a great source for listening to alternative versions of songs. I recently found that someone had videoed for YouTube a song I had collected and recorded - fair enough as I don't collect songs for them to sit in obscure archives.

As far as possible, I try to learn songs that I have recorded and learnt from source singers, but this process takes a lot of time and patience. However, I get an extra buzz by hearing someone sing a song they have learnt via oral tradition rather than the folk revival or the media.   

Tradsinger