I've led the Night Owl Hoot, the all night sing-along at Falcon Ridge folk festival the last 18 yrs or so. I've noticed a decline in the group-sing as folks gravitate to the all-night performance tents.
I lead the monthly sings of the Folklore Society of N.E. Pa, USA, and attendance had been good these 20 yrs, but few young people (I'm 58). There is a core group of about 25 songs that are sung a lot.
I perform in a bluegrass group, The Young Geezers, and when we encourage the audience to sing with us it seems to get harder every year.
My father's generation sang. They all knew the songs. I'm glad I was exposed to that. I think the spontaneous songs come from being around people who do it. Be the change.