Just remembered to actually note down the numbers again. Reach is sitting at 32,000 players on a Sunday, which is much higher than what it's population was staying around before GwG.
This is fairly good retention almost a month and a half after the day it went free, considering two free games have now come out since then, one of them being multiplayer.
Most population is still in Vanilla, same as before. Grifball's population is still high, MLG's is still the same, showing that continued DLC requirements hamstrung Anniversary and MLG from gaining any bumps from the free release. The highest population TU playlist by a country mile is Big Team Battle.
For comparison, Halo 4 is sitting at 9,500 players or so. It's hovered around that, so it seems the people playing Halo 4 right now are dedicated to Halo 4, even with Reach going free and Destiny releasing.