Yelp (YELP -10.9%) has plunged after delivering a Q3 beat and in-line Q4 guidance. The fact sales/marketing expenses totaled 58.5% of revenue, and revenue per new business account fell 11%, is likely raising fresh concerns about the operating leverage in Yelp's business model. Yelp noted on its earnings call its mobile apps were used on 8M devices/month in Q3, up from 7.2M in Q2. Total monthly unique visitors rose 7% Q/Q and 37% Y/Y to 83.5M - the Y/Y growth rate is down from Q2's 52%. Mobile site ad pricing is said to be similar to PC ad pricing, and ads will be rolled out on Yelp's apps by the end of Q4.