I recently started playing WoW again and I lost contact with everyone I used to play with. used to raid hard modes,etc.. Now kind of looking for new people to run dungeons, bg's and LFR's with. Server or cross server
Try openraid.us, pick out raids that are appealing to you, and hopefully you find some friends on the same wavelength as you.
Of course, you can always look on your server as well, but that can be a little tougher to find the right fit, and usually not as many options unless you're on a highpop server suited for your particular playstyle.