If Yelp is a sort of monopoly, then users' standards for its usefulness is a low bar, since they'll be comparing it to nothing rather than to a better alternative. (And before users have reason to suspect any foul play at Yelp - as they now do - it's more difficult for better alternatives to gain users' attention.)
I'm not sure there's room for a lot of competition for end users in a service like Yelp. "Review accuracy" isn't a metric that users are capable of measuring with any effectiveness - it's so subjective anyway. The only thing that will put one service above another is how many businesses have been reviewed, and Yelp has a huge head start there.
