AOL has purchased Sphere, a San Francisco-based startup whose service places links to related stories and context-sensitive ads on article pages, for an undisclosed amount. The advertising part is likely the draw; Sphere's clients include magazine publisher Time Inc. The new inventory should lengthen AOL's already impressive reach. In March, 170.5 million unique visitors, or 91 percent of U.S. Internet users, took in an AOL ad. Before selling, Sphere raised $4 million in venture capital from Hearst Publishing, Trident Capital, True Ventures and founder Scott Kurnit. [PaidContent]