Yelp's main database was a couple of MySQL clusters (several TB of total data) for almost all of the company's existence, but they've always had a penchant for architecture astronomy. The ad system had the most justification for novel data architectures historically (when I was there, it was generating and processing tens of terabytes of data per day via Hadoop/EMR, which was new and cool, and I'm sure it's at least an order of magnitude mode than that now).