I see a lot of solid advice here. I played fairly seriously in Cata, and then took a few long breaks, and when I finally got back into playing again, I found that literally all of my former WoW pals had moved on, so in a way, I was in the same boat as you are in now.
First and foremost, keep playing. You'll meet people, even if you are losing to them. Even message people you play against, not to be a jackass rager, but to congratulate them sincerely on their gameplay. As Shendelz noted above (side note, I learned to DK last xpac from his guide and got to well over 2k in 3s yolo-style, so I highly recommend listening/reading anything he has to say), don't worry about rating. Just get in there and play.
Play RBGs. I know, RBGs are not arenas, but you will meet people that way. I met my most regular partner in an RBG, started to run fives with him, and eventually ran regular threes with him.
Finally, and this may be the hardest, record your own games and watch them with a critical eye. What are you doing that is setting you back. Could you stop more CC on your healer if you paid more attention? Do you get faked all the time? Are you using defensives unwisely? How fast is your reaction time? For example, after eating a CC that you must trinket/IBF, how long does it take you to do it? This last question could be one of reaction time or of efficient keybinding.