There should be some system to send small gifts to users on some criteria. Gift should include stickers and other things. Stack over flow should email user and ask them to provide address to send a gift.
Criteria can be
User who pass some reputation like 700 or 500.
Who opens the site etc
Even on badges etc. Also it should not be that hard to get the gift specially stickers :) so it can be done.
Its a suggestion as I love Stack Over Flow <3