According to the Yelp company blog, they will now be rolling out with the ability for users to leave Check-In Comments when their friends visit an establishment.
This will allow users to leave instant reviews or tips when one of their friends goes to a resaurant, or simply "like" the restaurant they're going to. Or doctor, or retailer, or hotel - Although restaurants are the companies main draw, they also cater to any kind of business.
Check-in Comments differs from their similar app tool "Tips" in that it is more private. Messages sent are directed at the recipient and cannot be viewed by other customers or the business they're in. Like all Yelp notifications, you can be alerted to Check-in Comments through push notifications.
Yelp seems to be adding more social features after its IPO, which debuted last week at $15 a share. Although they are yet to turn a profit, they have still drawn high demand from investors. If they keep pushing social improvements, I think their demand will only increase. Last quarter they were being used on over 6 million mobile devices.