That's a really great question, and a lot of people have asked it. So we had Jason write a blog post to explain how we count visits.
TL;DR "We take the number of unique IP addresses seen in a 24-hour period as the number of “visits” to the site during that period. The number of “visits” in a given month is the sum of those daily visits during that month."
Love the announcement. But a followup question: if WP Engine has excellent built-in caching and optional CDN, why meter so strictly by visits?
It reminds me of A/B services that scale similarly: by the time you get up to 100k or 250k visits per month, you also get 50 domains and 1000 simultaneous tests and all sorts of unnecessary stuff.
TL;DR "We take the number of unique IP addresses seen in a 24-hour period as the number of “visits” to the site during that period. The number of “visits” in a given month is the sum of those daily visits during that month."
http://wpengine.com/2012/04/visitors/