I have completed the main Dawnguard quests on the Dawnguard side. I have been using Serana as a follower w/o any issues. I have also dismissed her at Ft. Dawnguard a few times and found her there later and had her follow me again. I dismissed her at Ft. Dawnguard so that I could complete the Vampire Lord perk tree w/o her sealing the kills or having a negative effect on the strength of my blood magic. After I got all the Vampire Perks, I returned to Ft. Dawnguard and she was not there. I tried the 72-hour wait to see if she showed up and searched the fort after every 24-hour wait. I also revisited all the Dawnguard quest locations I visited with her and did not find her there either. I also researched the vampire castle as well. I now return periodically to check Ft Dawnguard and she is still missing. I have heard that you can use PC commands to have her move to your location or go to her location but I do not play the game on a PC. Question to the Bethesda programmers, where has she gone and how do you get her back? Since her mother refuses to make blood cursed arrows, Serana is the only one who can replenish my supply when they run out. She is also a very able follower. When will this issue get patched on the xBox along with all the other DG issues (Lost to the Ages quest broken, etc.).

I have gone to going to the castle before dismissing her as she only travels at night out in the world and it takes her a long time to get anywhere. Note, you say you waited 72 hours, but that is how long before she will start to make her way home, it can still take 5 to 10 more days for her to get there.

Are you visiting the castle at night or during the day? She may only be up and about at night.