Population growth in Massachusetts was essentially flat last year, according to new US Census data that shows a continuing pattern of stagnation that many fear threatens the state's economic and social prospects. The census data released yesterday covers July 2005 to July 2006, and displayed a pattern evident since at least 2000: Nearly 50,000 people left Massachusetts over the past year, while about 30,000 foreign-born immigrants arrived. The state inched to its slender gain because 22,503 more people were born than died. "It's yet more confirmation that our worries about stagnation are warranted," said Paul Grogan , president of the...