eleonorang I'm rather trying to see how other people use it than to establish rules, but The Emoji Grammar would be quite an interesting book ?
@eleonorang @ClaudiaBefu In most of the texts/tweets/posts I've read I've noticed that many people put it after.
However, there's been a debate in Twitter, as the RAE (the main regulatory body for Spanish) argues that if an emoji is the last element of a sentence, the full stop goes after the emoji. But on that same thread some people (some of them translators and other language professionals) say that the emoji is a punctuation mark itself, so in the case of a full stop, the latter would be redundant. Others argue (and I agree) that this is an extratextual element and therefore should be put after the punctuation mark.
Here's the discussion in case you're interested and can read in Spanish: