Update: The issue seems to be around 50% packet loss.

I called Business Support and the person was unaware of the issue, so please also call in to report the issue so Comcast starts to realize how much of a problem this is.

I had the support person ping facebook.com from my modem which showed packet loss. I asked them to send me to the next tier of support which they did. (This ends up being a ticket where someone will call me back.)

The issue started for me around 12:50pm Eastern time.

Update 2:10pm eastern: There's another thread on the homepage: https://news.ycombinator.com/item?id=15637607 (update: 2:26pm the other thread was removed from the home page)

Update: I contacted Comcast Network Operations Center. They confirmed they're aware of the issue and working on fixing it, but didn't provide any more details.

Update 2:30pm eastern: The issue appears to be resolved.