30% is a significant drop. It's really unheard of. MySpace and Digg didn't have such ridiculous drops.

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.