We should probably avoid using Instagram in examples. It's such an insane outlier that it doesn't make sense to compare anything to it.
The overlap between Instagram and Tumblr users in the early days is huge. Both are targeted at young crowd with similar taste graph.
As a Platform Tumblr has great potential in spreading the content. Reblogs works just like retweet and a good content can potentially go viral. That also includes rebloging of promotional content.
I personally feel people who don't use Tumblr see this as yet another blog site. They have the best mobile app to post pics, stories, videos & even audio. After Instagram, I am slowly getting addicted to Tumblr's mobile app.