Compete.com blog has a post about the Demise of Google Answers and makes out of it - the triumph of Yahoo! Answers over Google. But I don't buy it.

Compete says Yahoo is the largest site on the Web - but I bet they also did a bit of work to drive traffic to Yahoo! Answers (perhaps knowing they could easily beat Google - which did not feature Answers prominently.
When you put out as many products as Google - some will fail - not everything can succeed equally. Maybe it's truer to say that Google lacked the focus and will to make Answers work and opted to concentrate on other projects. Even so, there were was a committed user base, perhaps small.
Because Yahoo has so much portal property it can drive traffic one part of itself to another - making it look, perhaps, more trafficked that it really is.
