"Active ISPs" is short hand for "number of /16 subnet ranges to post on this board in the last 72 hours."
It is not a perfect metric and does not account for number of lurkers (users who only read the board and do not post) or the number of users sharing an IP range (for example, all Tor users are considered one active user).
In the entire Internet, there are only 16,384 /16 ranges (also known as Class B networks), with 65,536 addresses per range. So, if /v/ or /pol/ has 3,000 ranges (active users), that means their posters represent 18% of the possible number of ranges on the Internet. Many ISPs only have one or two ranges.
[This page was updated on September 3, 2017. The previous version looked like this.]