We seem to be arround this topic every 6 months or so. I have tried ThaiNo1 and SweetSingles dating agencies and it DOES work if you go there and meet the girls.
Normally they are a lot more interested if the agency calls them up and tells the girls you are in BKK and ready to meet them. I recommend.
The pages Thailandfriends.com and faceparty.com are great. After chatting some time with the girls, they are willing to meet you. Mostly a big success. I would rate these girls as real students (with a very open mind) and some of them I would describe as semi professionals (the ones you get a date quite easy). Just look at their pictures and their desvription, easy to recognize.