San Francisco-Twitter has acquired Boston-based social discovery engine Spindle, the two companies said Tuesday. Financial terms of the buy were not disclosed. Spindle was developing local discovery software. According to Spindle, it is shutting down its service, and will relocate to join Twitter in San Francisco. Spindle had been backed by Polaris Venture Partners, Atlas Venture, Greylock Partners, Lerer Ventures, SV Angel, Broad Beach ventures, and Project 11.