Last Fm is the best way to measure the Audience size for a specific Artist, because of the major Social Music services (Spotify, Pandora, Last FM) they are the only one that actually publish statistics for Artists.
On this Graph, "0" is in 2008 for Spotify, mid 2005 for Last Fm, and 2005 for Pandora. So you can see here that Last Fm is a pretty big user group to be drawing statistics from.
On Last FM, Animal Collective has close to one million listeners, almost 55 million library plays and is #190 on the very excellent Last FM Top Artist Chart. The record itself comes out on September 4th, so it would be good to check on the statistics a month from now to see the rate of increase.