Huh, that is nowhere near the percentage drop I would've expected. I guess people actually use Tumblr for more than NSFW stuff.
And keep in mind that this could be the start of an overall downward trend. If 30% leave, then activity on the platform crumbles. Some of the remaining users notice the decline in activity and so they leave. Rinse, repeat. Once it reaches a critical mass, you can't stop the decline and tumblr is finished.
Or it could be a temperory decline and a new plateau is reached where tumblr can survive at.