A sample Facebook page is seen in a handout photo. REUTERS/Facebook/Handout Facebook’s vision of becoming a “utility” that offers activities to keep people online for hours could set it on a collision course with the Web’s giants. In recent days, the No.1 social networking company revamped its search engine and bought a start-up that some call a rival to hot micro-blogging service Twitter. It is also testing a stripped-down version of its service to boost growth overseas and is developin