I don't use either one.
I could do with less of social mileage and impact.
Facebook. I don't even know how to use Tumblr.
Neither to be honest. Facebook used to be great in the early beginnings. Now it's just for old people sharing and posting religious and political nonsense. But it still works great as an address book.
Tumblr was fun and exciting. But the community is now made up of mostly HAES activist. They even made a word for it called Tumblrina.
When you're over 18 and out of high school, the only one you should really care about is LinkedIn. Maybe Instragram and Twitter if you're actually doing something interesting with your life.
I don't use either.
Facebook. Tumblr is more artsy, but Facebook is straight to the point.
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