Yelp could easily roll out "AdWords" and absolutely kill it. It's insane they've dragged their feet so heavily on this.
Kill it with SMB money? Business like Yelp have always had a hard time monetizing mom and pop shops, and keep them repeat customers. The real ad money is not Uncle Joes' Falafal Shop down the road.
They're currently able to generate $10-$12 million in net income on $377m in sales. Their growth rate has slowed down drastically. Take their 52 week high (much less their all-time high), which is a roughly $6.4 billion market cap. Their best case scenario, ten years from now, is that they're worth ~$6.4 billion.
$200m in net income on that $6.4 billion market cap pegs them at a 32 PE (plausible, if not a bit rich, ten years out). To get there, they probably need - even giving them the benefit of expanded margins with scale - $2 to $3 billion in sales.
The only way they're going to exceed those types of numbers, is through using their equity on acquisitions.
Here they're sitting on a very rich stock market valuation, in one of the greatest bull markets the US has ever seen, and it's unlikely they're ever going to top their 52 week high. Selling is the obvious move.