multicast meltdown?

We've detected a large drop in the IPv4 multicast prefix count over the past few days. Anybody know what's going on?

Antonio Querubin
whois: AQ7-ARIN

At this point I haven't seen any problems, nor have customers reported any to me. I did see my multicast prefix count drop from about 8k prefixes to a little over 5k, starting on 8 January. The drop was quite abrupt.