The country filter is based on the IP geolocation information. This means that the system is not based on the information of the user on Facebook, but the filter-by-country system functions according to a user’s IP address. For instance, if a user has specifed Spain as his location in his profile but his IP corresponds to a Portugal location, this user won't be able to enter a contest filtered by Spain.
We have shown that when vote exchanging occurs, the majority of the votes exchanged come from outside the country in which the promotion is held. In other words, users who decide to exchange votes are not usually from the same country. One of the first and most effective measures for preventing vote exchanging is the option to filter the contest by country, so that only users from one specified country can enter a contest or vote for entries.
You can obtain the IP address for each entrant that registers, recruits or vote, and administrators will be able to see to what country this address corresonds. Sometimes the IP adress of a user does not correspond to any country and an error message is shown. To see the different error messages, check this link.
If you want to set up the country filter in your promotion, check this tutorial.