It could be interesting to get achievements for sticking stickers to different people, or for sticking different stickers.

Something like: "Stick a sticker to 10 (or more) different subeta users." and "Stick 10 different stickers to your friends' albums"

They could also mirror the "have all the (whichever set it is) in your album" to "stick (whichever set) to other users". They wouldn't have to be stuck to the same user, but it might be a nice incentive to also give out rarer or more expensive ones.