You know full well I would have to list and cite every single MMO that supported guilds and their relevant information which would be impossible as there is too much info. But lets start with:
WoW - over 10 million players
EvE - over 500,000 players
Tera - over 6 million players
Archeage - over 3 million players
ESO - Over 2 million Players
There is something important you are leaving out though for the MMO's you have listed, Lets put the 'social aspect' to side for a moment, the games you listed almost all, (if not actually all), have a necessity for guilds. Whether that necessity comes in the form of end game content, (which Elite D does not really have), such as operations, raids, flashpoints, 'H4 instances'. 'But that can all be done without guilds' I hear you cry, it can, to those that enjoy or chose to do so, but there are a great number of players that are adverse to 'pugging' and guild offer these players the chance to progess at end game things such as those listed and 'end game' PVP with players they know, trust and become familiar with the play styles of, (important for end game PVP and raid content for example). None of that is an issue in Elite D.
Some of the other games you have mentioned are extremely PVP and territory orientated, in these games, (EvE, Archeage, Guild Wars etc), the guilds are the game in these cases to a very large degree, again, not the case here.
Now, to the 'social aspects', I think we can almost al agree that Elite D needs some better social features, but these are more than achievable without guilds. An awful lot of the games you listed do have more tools for social interaction but most of the guilds in those games have an awful lot of their 'interaction' via 3rd party tools, such as Teamspeak, Vent and many more, there is not one thing stopping people from using these tools in Elite D. You can argue that a guild list or a calender will be some massive game changer, but be honest, it won't, and besides, you can have said list and calender on the guild website, something else many, many guilds do whether or not the tools exist in game.
Point I'm making is that Elite D is different to all the games you listed, player numbers are up for debate, but they are not a determining factor in whether guilds will work in this game or whether or not the tools for them are some sort of 'game changer'.