Yeah, I've known about this issue for a while, and am always looking to remove unnecessary plastics from my life. A good organization doing a lot of work around this is the Rozalia Project based in Portland, ME. I know they're trying to develop technologies that can remove microfilaments from the water. I've donated to them and would love to volunteer with them someday.
As of this year, I've pretty much committed to avoiding synthetics in clothing unless they're absolutely necessary. It's hard, because my hobbies (backpacking, and bike and canoe camping) are pretty gear-intensive. Not just clothes, but things like tents, sleeping bags, backpacks, etc. The easiest switch is using merino or silk for tops and baselayers (even though I love my heattech and capilene I probably won't replace them when they wear out). I've already more or less quit polar fleece; it doesn't perform particularly well and gets so pilly and stinky. Pants are harder, though - I don't mind wearing jeans or chinos for a quick hike, but you really can't take them on a multi-day trip, and merino pants I've found are not durable enough for anything but light jogging or yoga -- and not comfortable in hot weather. So for now I'll hang onto my old nylon pants and shorts and hope someone develops some amazing new technologies in the next few years.
Casual wear is easier. It means choosing 100% cotton denim rather than synthetic blends. It means avoiding poly in tops, dresses, and blouses. Which eliminates a lot of options and probably saves me money in the long run. No ponte fabrics for me -- I've decided it's not a need. However, the ecological impact of cotton, especially non-organic, is pretty high too. Linen and hemp are better.
For yoga and running this summer, I'm going to experiment with the hemp jersey "gear" from Conscious Clothing, a small company in Michigan. For everyday fitness, it seems like it could work, as long as it doesn't stretch out too much. I did try backpacking in a hemp/linen dress several years ago, but I don't think I'll do that again...