Too long? Don't want to read? If you freeze your filter, you will likely be able to filter Giardia and Cryptosporidium, but not bacteria.
Here's what we're looking at here;
The fibers in the Sawyer have pores in them. They become saturated with water, and the water can permeate through the fibers out the other end of the filter but contaminants can't. What this means is, after the first use there will almost always be water in your filter. There is humidity in the air, so the only true way to get the water out of your filter would be a kiln (obviously, don't do this).
So you have a water droplet in a porous membrane and that water freezes. If the water is in the pore, it will expand and rupture the fiber. At 0.1 microns, we're not talking about a lot of durability in the material. It will rupture. However, if the water droplet is not near a pore, it will freeze unrestricted and cause no damage. It's impossible to predict.
So, you might damage some of your pores and you might not damage others. You could also have more damage in one area of the filter than another.
What size are the contaminants? Let's look at the following chart:
As you can see, the 0.1 absolute biologic filtration system claimed by Sawyer is a complicated thing. I can't say I understand it completely. Sawyer claims 7 log (99.99999%) of all bacteria and 6 log (99.9999%) of protozoa are removed, but most harmful bacteria are smaller than protozoa like Giardia. Perhaps the smallest bacteria are significantly less common than the larger ones, I don't know. According to our chart, taken from the Grant County Emergency Response website, our 0.1 micron filter is enough to claim to filter all bacteria, or virtually all- 7 log.
So, if you rupture a pore in your filter, chances are you will still be able to filter protozoa, since they are the largest thing in the water at 5 microns. Your pores could expand or rip to 50x their normal size and still hit things that are 5 microns large, and the expansion of water while freezing is not 5000%. You will lose the ability to filter most bacteria, but since bacteria are only really prevalent in water that has been contaminated by waste, you can likely survive with no ill effects for an extended period of time without worrying about the bacteria in the water as long as you get your water from good sources, like springs. The protists are the common threat, and the reason filters considerably less dense than 0.1 microns are commonly used in the outdoors. The EPA only requires a water filter to go to 3 log (99.9%) to be considered a filter for protozoa.
Bottom line: it's probably ok to keep using your sawyer filter after it freezes. The chances of you getting Giardia or Cryptosporidium through the filter, even after the fibers have ruptured, is impossible to measure but very very unlikely- The Sawyer would probably still be better than a Brita filter (which can't even filter most microorganisms) and it's going to be way safer than drinking straight stream water.
Your immune system first filters everything through the liver and spleen, and then attacks any foreign particles with white blood cells, proteins, antibodies, and even good bacteria. Your body can likely handle the .00001% that the Sawyer misses, and whatever it misses after having been frozen.
So, if I were you, I would keep using your sawyer after it freezes, but I would try hard not to let it freeze.