I met my first serious bf online, we were together for three years and engaged in the end. Ok it didn't work out but he didn't turn out to be a serial killer. I also met another guy on a dating site when we'd split up, and that was ok too. Just got to use your common sense.
I used to be on a dating site where we used to have regular meets at pubs/clubs. There were loads of people that went and you could take along friends if you wanted. As it was always in a public place with lots of people, not just a one on one it was much safer. I have made loads of friends through the meets, male and female. Infact I met one of my best mates at one of these meets nearly 3 years ago.
Common sense really comes into it, always a public place and always make sure someone knows exactly where you are going or take someone with you.