There will be a lot of player churn at the beginning of any MMO, especially one that has a relatively short grind to 50, and relatively limited content post 50. I mean, don't get me wrong, if you go through end-game at a pretty normal pace, it will take A LOT of your time, but at the same time its pretty easy to shortcut through a lot of it and easy-mode your way to the upper echelons of the game.
That being said, I still very much like this game. I'm currently working on T5, and got my first X mode primal weap this week. My static isn't the greatest, but I enjoy hanging with them. I think I've left one linkshell (Hard Mode Group find, lol), and I've had to delete 5, count 'em 5, people from my friends list (out of roughly 150). I think what you will see is FFXI, EQ1, Ultima Online, etc type of people will eventually get disenchanted with this game if they are looking for a replication of that experience. The 54 man raids of EQ and the 60+ man raids of FFXI are NEVER going to be present in this game (IMO). It will be up to players to play together. For instance, my FC does 1 night a week outside of our single Coil group where we do Ultima, farm crafting mats, grind myth, get Titan HM kills for people's alt relics, etc. Probably 1/2 the static members show up and some other people from the FC get to have some structured fun : D
The 8-man clique thing . . . I kind of have similar problems in my FC. Not that the 8 regulars in the coil static are the best of friends and only group with each other, but that the members who couldn't commit to our times or joined late or didn't have their Relic are feeling left out. However, its mainly the people that are unwilling to PUG to gain experience, and the people who sit around crafting all day, not working towards X primal accessories or working on other jobs to be more diverse raiders. Not that that is what you HAVE to do to have fun in this game, but it certainly facilitates the likelihood you will become a raider.
I actually do come from 12 years (on and off, probably 7 years of playing on a weekly basis) of Everquest, played from the 2nd expansion to the...12th? Then led a progression raiding group on a time-locked progression server in the 9 months proceeding me leaving that game for FFXIV. I quickly realized FFXIV was NOT Everquest, and I would not get the same sense of being lost in a giant world I could never hope to fully understand or explore.
My opinion is that, given some time, a nice group of people will eventually settle in and call FFXIV home for a while. The people who are really looking for an open world endless possibility game from FFXIV will eventually realize that it won't happen, or that they can unsub for a few months while they wait for another massive plate of content to devour, and we'll get waves of population at each patch and expansion. I'd predict at about 12 months you will have a friends list that you can expect not to dissipate quickly, unless SE introduces a Competitive Eater class or something.
I know its sad for someone who had 20-50 friends from 1.0 or FFXI to see people leave, but just ask them where they are going, plenty of people will stay here to keep this game alive, go where you will have fun again - it will be a win-win for everybody!
TL;DR: If your friends list is full of people that loved FFXI or EQ1, they are probably going to leave to go play a game like that. FFXIV is not a game like that.