Can I use it for quilting backs like flannel?

Now I'm not too sure if you would want to use it like flannel. I would think that felt would break down much faster than flannel. I could be wrong! Let's see what others have to say.

I wouldn't do it. That's just me.

I don't think I'd use felt. Have you tried washing it?

I wouldn't either.

No, I wouldn't. Felt can pill up to, so I would stay away from it!

No, I haven't washed it yet. Guess I better just sell it because it will just take up space. Hmmm. Thanks.

Is it wool or poly felt? Craft felt or better than that? It probably wouldn't work for quilt backs as the others have said, but it might be fine for the backs of coasters or other small items that don't need a lot of washing.

Felt doesn't hold together through washing as well as a woven product.
One neat thing though is you an cut out shapes and the edges won't fray. What kind of felt is it?
Felted wool is used to make a ton of neat things. Just go to the Etsy web site and type "felted wool" in the search and you will see a lot of great ideas.

make some microwave rice bags
for those acky joints