With revenues that exceeded $1 billion for the first time in its 21-year history, it’s no surprise that Sapient Corp., based in Boston’s Back Bay, is back again as the top services company in the Globe 100, a spot it has occupied two out of the last three years. The biggest driver of Sapients’s 2011 performance was the company’s SapientNitro marketing services business, which grew 33 percent over the previous year and accounted for 67 percent of revenues in 2011.