I have a similar issue, also in King County. The sagging line is from utility pole to utility pole and NOT a drop line. In this case the line is only about 3 feet from the ground. This is in Unincorporated King County and NOT on our property. The line has been sagging for weeks and seems to be getting worse. The cable is a fiber optic cable, so frankly I'm not sure if it belongs to Comcast or Verizon. How can I report it?
Call your local emergency services number (the non-emergency number, not 911) and let them know about the hazardous situation. The police will typically respond and contact the proper utility.
It may be that someone was monitoring the forum or the timing was just right, but the sagging wire has now been fixed. I'm still not sure whose wire it was, but thanks to whoever fixed it.