I got curious what country is most Facebook. Here's the Facebook penetration in different countries according to their advertising tool with setting "I want to reach people age 18 and older in X". I should be noted that these numbers fluctuate and may not be entirely correct. For example "I want to reach people age 13 and older in the United States" gets you ≈ 22,850,940 people but "I want to reach people age 14 and older in the United States" goes up to ≈ 25,731,540 people.