The way I understand it (IANAL) is that you need to protect your trademark against people who uses confusing namings on things that isn't you. In this case "tum

The way I understand it (IANAL) is that you need to protect your trademark against people who uses confusing namings on things that isn't you. In this case "tumblr" clearly and explicitly refers to tumblr.com. If you had to make sure that no one uses your trademark to talk about you, wouldn't that lead to a very strange place?