What is the 100 day challenge on facebook?

Well, there's 2 meanings.

The first one.

The 100 day PICTURE Challenge.

That one means you have a 100 days, and each day there's a new type.

You post that in its own album. So at the end there will be 100 photos.

The second one, it's a weight loss program for 100 days.

(see related qestion)