I logged in and checked Dungeon Finder, since there was extra loot for healers, I queued. Note, I'm not real social butterfly, I don't "/g hello everyone" because I don't care if people know I'm around or not. So, I'm queued up ready to get my satchel of exotic mysteries when I get a group, and jump into a Random group filled with players from my own guild. Apparently, their healer left and I got randomly selected to replace him. We all had a good laugh about it, especially since they hadn't asked me to queue with them in the first place. It's not like they wouldn't have asked me, if they knew I was around and looking for a group. I least, I that's what I tell myself before I go to sleep.
The Dungeon Finder's new bias which matches with players from the same realm first is apparently to blame. I've noticed a lot of people from the same Realm in my groups, and I think it's a great new feature. It's amazing how many of these players whisper me, "hey you guys are level 25?" So many new people looking for a high level guild.
If meet some random player from your realm and they are good, add them to your friends list. These people could fill in gaps in guild runs, be it dungeon or raid. Make a note, good deeps, great tank, whatever, so when you are looking to fill in a run, you'll have a list of contacts to check.