You can send customer reviews by visiting other players and tapping on the mailbox in the top right corner on the street. Once you have collected enough reviews, you can exchange them for a special gift. You can see how many reviews you received by tapping on the mailbox in your own restaurant. If you tap on the different reviews, you can even read them! Lastly, after tapping on the mailbox, you can use the "Ask for review" button to invite your friends via various social media platforms to leave a review for your restaurant.