Given the URI-centric approach of Activity Pingback (and Activity Web) I'm leaning towards /username over @username to mention/reply-to users on Converspace. This works very well because it's just a relative URI and lends itself well to the Federated Social Web by allowing you to mention/reply-to someone on another network with simple prefixing: some-other-social-network.com/username or another-social-network.com/users/username.
- "/ is the new @." - Dave Morin
- Slide #37 of Social Network Supermarkets and How to Defeat Them - Chris Messina