Recently moved to WP Engine. Couldn't be happier. Wordpress is awesome but really sucks when you have to host it yourself.
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."
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.