Population Migration / Travel
Less than 8 years after the formation of the colony on Luna, there are 1000 people at one time on Luna. At this time there are a total of only around 1400 people off of Earth, so it is seen as quite a momentous occasion.
Overall, this is not too much of a marker. Without genetic augmentation that is currently unavailable, Luna will not be a colony so much as a place to work. The sole reason for the large population is the Helium3 refinement process, as well as the superconductor extrusion. Without these activities, nearly all of the current residents of the moon would leave. On average, they only stay for a year or so each, but there has been construction of a shallow spinning bowl, which would allow for 0.3 g, often seen as the minimum.