I think the higher churn replacing players creates a different social aspect regardless of game features, but for this thread here's the real issue with 5 mans that's changed since vanilla/tbc and wotlk/cataclysm:
The time requirement has dropped considerably because the instances are streamlined more. Less time = less social interaction.
There is no real negative to dropping a group in the LFD system. In vanilla/tbc it could literally take hours to find a dungeon run. You would teach players how to play if needed because it was easier than trying to find replacements. In LFD the debuff isn't punitive enough to force interaction between players like before. It's currently quicker to drop than teach a new person how to play.
Encounter design is overall easier. I know people will argue that cataclysm is hard, but compared to the original and tbc trash in the instances are easier overall. No huge amounts of CC requirements and class design makes trash easier every patch. Think back to how many times you used to see lucky charms on trash in vanilla/tbc compared to today's trash? Easier encounters = less social interaction.
Finally, they are trying to match same servers first for 5 mans before going cross-server which is a good thing in all fairness and destroys this argument some.
The more Blizzard streamlines the game the less social interactions you'll find. Requiring a player to actually form a group or interact with a player is against the streamlining process that Blizzard keeps working on. One example is the leveling process: with the increased experience gains, it's now easier to gain levels and come back to a harder quest than to actually group and do it at the right level/time.
The community dictates the social aspects of the game too. On my server at least if you ask a question in trade chat/orgrimmar you get a response of "check wowhead." Before wowhead it was thottbot. The outside websites have also contributed to the decline of the social aspect since people don't have to talk about things, but instead just look them up.
Facebook, Google+, Myspace, etc. I know people who had accounts and would log on just to check up on their friends back in vanilla/tbc. There's no need to do that anymore with the social networks being free and available to everyone.
More complex or time consuming = More social interactions. Think about it.