I don't think it's a trendy thing, I think the etymology of the word "nerd" has evolved to mean that you have a deep and detailed understanding of a particular subject.

Perhaps Tumblr, in the eyes of teens, has a steeper learning curve than spewing out pictures with Instagram/Snapchat and being adept with the platform indicates you have better technical capabilities.

Either way, I don't think it's a bad thing that the word "nerd" has changed in this manner.