FB’s brand is suffering. They understand this themselves obviously. It’s best to liberate new products from the word “facebook” and make it appear as a seperate entity. That’s basically what this is.
With Google/Apple/Microsoft it's quite straightforward, everything they make is clearly their style, their brand. This is not the case with Facebook's properties, they don't have a unified design language or branding. So adding a unified splash is a step in "marking it's territory", I just hate the feeling of that.
Although, that splash has helped me reduce my usage of FB apps, so maybe that's a positive.
"Your honor, we can't possibly spin off Instagram, it's now part of the core Facebook product!"