Assume the best, not the worst.
My parents are a bit internet clueless in some respects (like, my parents didn't know you could get porn on the computer just by typing in the word "porn" or even less obvious terms). In other cases, they're overly paranoid (e.g., can someone sneak in a camera to watch you over the internet?). They're just not as savvy, even though they use the computer all the time. For silly stuff, you know... like FB.
Anyway, I got things like a cat (from my grandparents) and pierced ears (from my aunt) that my parents didn't allow for me. The cat I couldn't keep, but the pierced ears, eh.
Just explain your rules to your ILs (or better yet, have your husband explain it) and if you like, explain why. You have good reasons for not getting your child involved in un-monitored (and for-profit) social media. Stick to your guns. And if your kid won't hate you for that, she'll find some other reason.
