Last September I've travelled alone in western Europe for about a month. I was very worried at first, and read lots of stuff in forums, especially
Thorn Tree Travel Forum - Lonely Planet, but it was a great experience overall, met lots of interesting people and it definitely helped increase my social experience. After a couple weeks I felt so easy with people I had many random conversations with strangers and after 5-10 minutes they would start telling me some really private things about themselves and some introduced me to friends and invited over. And I'm usually a rather introvert person.
The biggest regret though is that I'll likely never see any of these people again.
I mostly stayed at cheap hostels - not only to save money, but also because the cheaper it is, the more social the place. In paris I was in a small room with 6 people. Beds even had no sheets and there was only one restroom on the floor. But the people were very friendly and we talked a lot. I usually stayed at each city for 2-3 days and then went on to the next one.
I think traveling alone is definitely the best if you're single!