Complex sentence: San Francisco, where Yelp was founded, remained the most active city as of 2008, though there was significant adoption in 18 metro areas including Boston, Chicago, New York, Washington, D.C., San Diego, and Los Angeles.
Split into simpler sentences: San Francisco, where Yelp was founded, remained the most active city as of 2008. There was also significant adoption in 18 metro areas including Boston, Chicago, New York, Washington, D.C., San Diego, and Los Angeles.