This is their statement.
"
I do believe you’d be very pleasantly surprised with not only the number of players who jumped on during the free play period (holy moly lots! by golly it worked!), but also the amount of Halo 5 players each day and month - this isn’t something we’ve talked about yet, but I am rather happy to share that even before the free play period, Halo 5 has had the highest monthly active players for a Halo title since Halo 3.
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 
I’d have to get approval from the powers that be to say more in terms of specifics, but you should rest assured that together, we’re quite a strong army."
They also did this population count before the game was free. But while this may be a sign of hope that Halo may one day regain its frontrunner status, it isn’t very clear. First of all, how did they figure out the player count increased? The game started off as #1 on the most popular page for a few days. It dipped into the top 10, and then went to around 11 to 12th place. Then just before it became free it was around 18th place? Its at 6th place now, but probably because of it being free. Perhaps the site is very wrong, or 343 is wrong.
Anyways, what exactly does this statement mean? highest monthly active players for a Halo title since Halo 3. Besides the extreme fanboys who don’t allow any criticism to my favorite franchise, we all know that Halo 3 was the peak of Halo in terms of players. It was in the top three most played for 3 years. What this statement actually reveals, is that Halo 4 must have done worse than Halo 5, by a longshot, and we all know why. Clearly, Halo 5 improved population-wise to 4. But what about Reach? That seems very strange, considering how Reach was pretty popular until 4 came out. However, Reach took a BIG hit once Black Ops came out, which is the peak of COD. Anyways, does the statement mean that Halo 5 had more players then Halo 4 and Reach in just one month? Was Halo 5 more popular than 4 and Reach through October to now? Was the average population of Halo 5 from October 2015-July 2016? higher than Reach and 4? This probably makes sense. But you can’t compare Halo 5 to Reach. Reach came out a month earlier, so even if Reach decreased at the same rate as Halo 5, Reach would have a smaller population because it would have decreased for a month more.
In either case, this achievement is a glimmer of hope, but not a very big one. First of all, we don’t know the actual numbers, the methods they used to find this, and we must remember we have a long time ago to make Halo an innovator again. Halo 3 had a gigantic population. Even if Halo 5 had the highest number of players since 3, it doesn’t mean much because whatever numbers 5 has, it probably wasn’t even close to 3, but somehow beat Reach and 4. Again, we can’t figure out more due to little information behind the statement.
I want to know how you guys feel about this, and PLEASE 343 if you see this, all I ask is how exactly did you do this, because I don’t understand how you compared the game to Reach.