I've been a solo player ever since I started playing again back in cataclysm. (Was forced to quit during wotlk due to my computer breaking down)
When I first logged onto my old toon and looked in my guild I was so extremely disappointed. The active and social guild it used to be was completely empty. I was the only one online. Goldshire was also empty (which used to be filled with twinks, inexperienced newbies and high and experienced mains).
Since then I played solo. First few weeks was OK, I was busy levelling a rogue and was motivated to hit level 85 as I had never hit 80 during wotlk. But then it became quite boring. People were joining guilds because of their level, not the social aspect and I basically lost the will to actually be social in WoW.
Even now, after I found a tiny guild with a couple of active players who oftenly talk in gchat, the games feels so... empty. Also everytime we try to recruit, people either leave the guild in a couple of days, or never log on. I believe I'm the latest newbie to the guild, and I've been in it since start 2013 simply because nobody joins a guild which can't do anything immediately, rather than just talking with eachother.
I know this is more related to WoW, but it's starting to happen to other MMOs aswell. Guilds are quiet and empty, local chats are extinct and everything is about ranking and queueing.