UPDATE: You can now @mention people. Type @username in a post or comment and it turns into a clickable link to their profile, and they get notified. Use @friends to ping your friends, or @everyone to reach your friends and followers. Your privacy is respected: a friends-only post never notifies people who cannot see it. Two more: adding a friend now follows them automatically once they accept, and the Add friend and Follow buttons are the same size now.