On the one hand, I see no real problem with having a separate, private, account for social media. You don't have to do -everything- with your partner. On the other hand, him going off like that definitely shows it was more that just some friend/hobby thing and definitely something more nefarious. I'd keep a close eye on him, if I were you.

I mean, you already have good reason to suspect shenanigans, so you could just tell him this is a hill you're willing to die on, so to speak. Or... you could break out the ol' catfishing kit. Make a fake profile of some generically pretty lady using whatever stock photos you can find and change up a bit so he can't reverse-image-search them (y'know, if he's that smart/suspicious) - Don't make it too obvious that it's a fake, and don't try to add/follow him right away - Let him get more comfy and forget this confrontation somewhat first. Then, once added, see what sort of nonsense you can find, and get evidence to confront him with, to back up when you confront him again (and if it's that serious, for use in any divorce proceedings). But, like, I mean, that is a *lot* of effort just to confirm what you already suspect - Your husband is an ass and doing something sketchy that he feels would otherwise cause problems for him if you found out the exact nature. So, do you really want to continue putting effort into this relationship? I guess that depends on everything else in your shared lives.