Wall cancer is a widely used facebook term used to describe members of groups with an uncanny ability to scare other members off group(s) simply by posting the most uninteresting and boring shite imaginable. It is also used to describe wall posts on fb groups. In many cases group members have even been known to leave groups due to copious amounts of garbage being posted by dolts with no sense of humour or purpose other than to amuse themselves.
Facebook Group Wall:
Higz: It's been quiet here today, where did everyone go?
Vanessa: Thomas turned up on group last night and posted a photo of a woman with grape hair and soon after everyone said goodnight and left. Some members even mentioned they would be leaving group permanently.
Higz: Great! didn't he post that dumb photo on group last week?
Vanessa: Yep, it was the same one. I think we should toss Thomas off the group, the guy is a wall cancer, and has actually managed to scare off a third of our members
Higz: Give him the boot, V