That said, there are definitely some situations where this would be considered uncouth. Eating with in-laws or at a nice restaurant come to mind! In these situations, we opt for holding the shell in the fingers of one hand and using a fork to scoop out the meat. We lose our beloved pan juices, but it's definitely a neater way to eat.
Using an empty shell as a tweezer to pick out the mussel meat never occurred to us until we did a little research for this post. This strikes us a perfect compromise between slurping and using a fork, and we'll give it a try next time we have mussels.
And anyway you go, we think mopping up the leftover cooking broth with a little bread is a must. You just can't let that go to waste, proper etiquette or not!
Related: How to Clean and Debeard Mussels