Tumblr functions quite differently to Twitter; my take on it is that Twitter is more or less essential for certain types of communication, and it’s where celebrities broadcast their voices. Tumblr, in contrast, appeals to niche groups and is good for posting long-form mixed media messages.
As @zenvelo said, I don’t think you can extrapolate trends from Twitter to Tumblr or vice versa. However, these kinds of numbers can surely be checked, if you’re interested in looking, When Yahoo bought Tumblr a few years ago, there were concerns that they would break it or shut it down, and although they did break a few features, I don’t see a large negative effect on the userbase – but, of course, I see only my corner of my niche group.