[philipcristiano]

Can you explain the reason for this though? It seems like the site is talking about the actual website Tumblr, I'm not confused about the brand in question. I'm not well informed from a trademark stance but it seems like as a consumer (and this response page) this site is not confusing the trademark at all. Was it previously using logos/wording saying that it was Tumblr?

[jfarmer]

Although confusion can figure into a trademark action, it isn't the only thing that can. The rule of thumb is that if someone is using your name you should at least attempt to enforce it, lest someone later try to use that lack of enforcement as evidence your mark is no longer "in use." You can read this as "jfarmer's rule of thumb" if you'd like