They are short, memorable and don’t have “butt” in the name.

I currently have halibut baking in the oven with garlic butter, and butternut squash soup.

I love a good dumb butt joke, but not everything with the substring butt in it is automatically funny or reminiscent of butts.

Correct, it has to be at the end to be funny.

Halibut is already a terrible name for a social network but if it wasn't it may indeed be a little bit bumular

(although in my accent at least you don't really end in a butt, it's more a "bit", maybe you could get away with it)

But you are also not trying to convince people to try yet another social media concept named “butternut”.