I wonder about geographical and global (that is different countries and languages) effects. In my experience the 'hehe' or even 'eheh' is a much more common European statement than the 'haha' of the Anglophone world. I know Facebook restricted themselves to the US, but a global study would be very interesting...
It doesn't say whether Private Messages were analysed, assuming not.
What's frustrating about PMs at the moment on FB is that only on the mobile apps is it easy to use emojis. The smiley-face link on the messenger.com interface & the facebook.com domains only provide a choice of "stickers" (which are awful). So unless you know the keyboard shortcuts off-heart you're stuck using plaintext.
Stability is a great way to counter this effect. If a county goes up and down the cancer rate chart then it's probably statistical noise. However, if it's steadily at the top every decade for 50 years that's likely to have an cause. Even if it's just an older than normal population.
"You might have noticed that we cut the plot at 20 letters, but as with any behavior on the Internet, there is a long tail of laughter lengths. Our automatic regular expression parser gave up after trying to get through a haha over 600 letters long! Computers have a long way to go before they can truly understand the human condition. We weren't laughing that day."
And by emoji, do they mean the Japanese ones? Because smileys and emoticons existed long before emoji...
Maybe the facebook users are different than the internet crowd I know (mostly forums).