The impact isn't necessarily overt in this type of situation. Not just talking about Tumblr/Yahoo, but in general -- when evaluating an acquisition after-the-fact, there's more than just overt branding to consider. Think about amount of support from the parent company, rate of feature rollouts, ad quality, company culture, turnover / morale, hiring bar, internal transfers, etc.
Despite being an M&A buzzword, synergy is really important and doesn't mean meddling or interference. With Instagram and Facebook, consider that FB already had massive expertise in growing a successful social network, and ditto regarding infrastructure, as well as monetization. FB also advertises Instagram in-app for free. And Instagram is "cool" so there's major motivation for talented people at FB to do an internal transfer to Instagram.
Similar situation with YouTube and Google. e.g., Google was undoubtedly able to increase YouTube's growth rate by virtue of being the top search engine and tightly integrating all YouTube content into Google search.
With Tumblr and Yahoo, it's just not the same situation. They aren't headquartered on the same coast; their audiences don't overlap, which is especially bad for monetization; their products don't naturally interact in any way; the parent company has no experience with running a user-generated content property of this size; and the parent company has been viewed as being in decline for many years.
It was. Differences: Instagram's user base was still growing, mobile-first. Tumblr has plateaued, and still feels more like a web thing in a mobile world.
But anyway Yahoo didn't ruin Tumblr and that's a thing to be grateful for.
