How do I translate faire boule de neige into English?
The Collins Robert bilingual dictionary gives to snowball for faire boule de neige. This is a good place to start, but there are other possibilities. They range from the plain to the fancy, so you can choose the one that best fits your context and writing style.
Here's my list of suggested translations: catch on fire, gather momentum, grow rapidly, have a ripple effect, have a snowball effect, snowball, spread like wildfire.
So whether it's good or bad news, you now have more than one way of expressing it.