Trying to emulate the popularity of Web sites like Facebook and MySpace, hundreds of college alumni associations have begun to offer their own online social networks, seeking to stake a claim on the computer screens of current and former students, especially young alumni. Many of the sites have struggled to attract alumni and to keep them interacting with the devotion they show to their online profiles on other networks. That makes the sites less useful to colleges, which want to foster closer ties with alumni and keep tabs on their whereabouts for fund raising and other purposes.