The UI should be more proactive in encouraging people to write. For example, when you haven't sent a letter for a while, e.g. a month then finally do, you get an achievement which is great BUT you don't know until you do. The UI should be proactive and suggest it on the users name that a potential achievement is possible, perhaps with an icon, to encourage someone to write. When they 'send' it should unlock the achievement and then ask if you want to use the new stamp, e.g. LongTimeNoSee stamp.