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he way Brainify tries to set itself apart, however, is in its exclusivity, Mr. Goldberg said. Unlike general-interest sites like Delicious, Diigo.com, and Ma.gnolia.com, Brainify restricts membership to those with college e-mail addresses. And rather than link to fried-chicken recipes or the latest YouTube hit, users are likelier to bookmark animated illustrations of particle physics or explications of John Donne’s poetry.

“To artificially limit the size of the community, it artificially limits the use of the product,” said Mr. Scheinfeldt, who is also a history professor at George Mason.

Limiting access to those with college e-mail accounts could create a “walled garden,” excluding those not traditionally considered to be part of academe, said Melanie McBride, a new-media consultant who teaches part time at Centennial College, in Toronto.