Posting in a reddit thread about Halo 5's position on the most played list on the Xbox One, Halo community manager Andy Dudynsky (bravo343) stated that,

Halo 5 has had the highest monthly active players for a Halo title since Halo 3.

This is the first time we have heard of this, and while Dudynsky stated he can't give specific numbers without approval, Halo 5: Guardians has seen an increase month over month as well.

Also exciting (and not yet discussed, either) is the increase that H5 has seen in players each month over the past few months (we just got done looking at June numbers, which are once again higher than the previous month). As you can imagine this is pretty darn exciting to see, and this thread seemed like an appropriate place to share this news for the first time :)