According to Alexa, Reach is determined by the number of unique Alexa users who visit a site on a given day.

But also, according to Alexa, when related to (say) Facebook, Reach also measures the number of users-per-day expressed as a percentage "of all global internet users measured by Alexa".

Note from Alexa - traffic data are based on the set of toolbars that use Alexa data, which may not be a representative sample of the global Internet population.