People get busy. And many of them won't do it if it's not easy.  It's best if you offer them the specific links that go straight to the review site, (give them the option of Yelp, Google or Facebook) with detailed instructions (will they have to log in?  What if they don't have an account?) and follow up. 

We follow up just once if they don't leave a review, and ask them to fill out a customer satisfaction survey as an alternative.   Then we have testimonials we can copy on our site or our brochures, even if it's not a review in Yelp or Google or Facebook.