I realize this is a long-gone thread, but I'm going through a situation that fits the topic perfectly. I'm actually planning to take a cruise solo (more politically correct than "alone") this spring. Carnival Cruises, the line I'll be using, is quite good at accommodating solo travelers, from what I heard. They sit them together in the dining room, and there are plenty of social activities, such as organized sports games on the lido deck and dance classes in the nightclub (I'm a good dancer, so that'll give me "leverage" in meeting women). So while I expect to do at least some things by myself, I don't expect to feel lonely 100% of the time. Plus, since I smoke and the smoking areas on Carnival's ships are quite limited, enjoying a cigarette while looking at a pitch-black sky full of stars provides a wonderful mingling opportunity.
Now, being a budget cruise line, Carnival makes up for the low fares by giving you the prison shower treatment when it comes to things like alcohol, on-board merchandise, and shore excursions. That's bit of a downer, but I'll just bite the bullet and adjust my vacation budget for high alcohol and excursion prices. I'm choosing Carnival for its fun atmosphere, and the respectful and accommodating treatment of solo travelers.