It's a no brainer really
"The fact that people use social networking services for communication puts them in a mindset that is less receptive to advertising than when they're using other Web sites like Yahoo or Google," said Karsten Weide, program director for digital media and entertainment at IDC."
I don't think it's that. I think online ad banners are in general shit and continually fail. Could be quite indicative to other forms of advertising- eg tv ads, radio ads, print ads- people just ignore and skim over them.
Maybe it's time to move on?