Sure thing it is the biggest network if you conveniently just count every Windows 10 user with pre-installed Xbox app as a user, no matter if they use it or not.

Monthly Active Users... 

Xbox Live is in Autostart by default and uses the account data you create or use when installing Windows 10. So I bet there's a whole truckload of "monthly active users" who never actually take a look into Xbox at all.

Xbox doesn't register you as an active user if there's no account and your Windows account doesn't bloody count. 

There's no such thing as a windows account. When installing Windows 10, you create a Microsoft account. Guess what, you can use Xbox Live with just that account! Ok, you'll have to start the Xbox Live app at least once to create a GamerTag and all. But it's still sitting in the background if you don't. I can't prove it, but I bet my ass Microsoft ist counting those people nonetheless. I mean the article even says:

"Now Xbox Live spans both Xbox One and PC, and using one social network to link both user bases is “powerful.”"

On that neat little image Windows 10 is pictured wit 500 million MAD. So that's the point where I'm lost, what the heck does MAD even mean? Tried googling it, failed miserably. But anyway, there's really no point in arguing about this. It's pretty obvious that other networks like steam have a bigger userbase and you can only get to the conclusion that Xbox Live is the biggest if you go ahead and stretch the truth to your liking. It begins right at the beginning by describing ist as "the biggest gaming social network" instead of just a gaming network.

That said, it's not like it's in any way important to have the biggest network. This is all about bragging rights and Microsoft bends the truth a whole lot for those. They even once described Windows phone as a success. =P

Even though I feel the need to state it, I will not. The highlighted will suffice as proof of you having no say whatsoever. Also, Windows accounts refer to the log on and log off part of it. The MS account is just the name for the online services. 

Hence why they mentioned XBL being spread out over both platforms. People who use the Store are counted as users as well. 

"MAD" translates to "Monthly Active Devices". Also, gaming network and social networks are different. 

Unless you want to say Nintendo has a social network. MS is along Sony the only other company operating a gaming social network. Arguably, Steam and Twitch might count, but it depends on the criteria used. 

Streaming but from one users console to the second, they can simply be sat watching your live stream then be given control from their own controller.

Also, for the record, if you are somewhere else sucxh as at a relative/friends house and they have a PS4, you can add a new user and login with youre own details, and as long as you stay logged into your account you'll have access to your own games, cloud saves etc, without ever activating the second system to your account. When you're done you either delete the account off the second console or simply set it not to save your password so if you go there again you simply re-enter it and have access to all your stuff again.

So it doesn't really work as I expected. Unless, both users, in any case, are able to use the games on the second system. 

Well both users can play the games on the second system, both the games that were already on the system and the games for the added account, you just have to login to your own account to play the games you own on the second system.

There are other ways of sharing the content via sub accounts too but if you just want to play your own games when not at home, that's how you do it.

Both Microsoft and Sony approach it in diffent ways but the point is it's possible on both, it's also possible on Steam, too, and IMO, the best version is the one Steam has implamented.

When it comes to gaming, you just cannot fight Steam. They are the best. But this one sounds more restrictive...