Singles cruising through an agency is a pretty good way to go about single cruising since they make it easy to actually find the singles. I used singlescruise.com and was impressed with all the details they put into it. I haven't found another service that offers all that they do to meet other people before and during the cruise.
I went on the New Year's one last December and there were a lot of 20-something singles on it. I heard they Halloween one was good for young singles. I'm also going on the 4/11 one. And I'm going on the 7/31 Alaksa on Diamond Princess, but that's not a single cruise, but I'm going with my single girlfriend, who is the same age I am (26)....and I think we're babes.