The amount of money spent on advertising by local businesses is amazing. The opportunity for Yelp (YELP) remains largely untapped as the company expects to reach $700 million in revenues this year while the Yellow Pages collect $7 billion in ad revenues. With the total local ad spent tapped to reach $151 billion, Yelp has plenty of growth opportunities. Maybe this is why David Einhorn of Greenlight Capital bought 3.2 million shares worth over $60 million. Investors should buy alongside him. Disclosure: Long YELP