As for why collaborative filtering didn't take off, I can offer a few reasons

… As for why collaborative filtering didn't take off, I can offer a few reasons. One is that honestly, the vast majority of people don't rate enough things to be able to get a lot of signal out of it. Internally we had great coverage in SF, London, New York, Tokyo, and Zurich since Geo had teams in all those places and we pushed hard to get people to rate everything, but it dropped off in a hurry elsewhere. The data eventually fills up, but it takes a while. I'm told we had 3x the volume of new reviews that Yelp had at the time, but Yelp mostly only covered the US, while Google Maps was worldwide, so density was quite low for a long time. It was probably 5-10 years before I started hearing business owners consistently talk about their Google reviews before their Yelp reviews. Another thing is that people are really bad at using the whole rating scale. On a 1-5 scale, you'll probably find that 80% of the reviews are either 1 or 5 stars. Even more so in a real life situation where you meet the humans involved. …