try kumuka..... organised tours all over the world, small groups, lots of singles travel around the world with this reputable company - travelled with them in central america last november and thoroughly enjoyed the experience - few singles on the trip too...
Did a trip in Australia a few years with Contiki , Was kinda worried that it would be full of boring people with no imagination. Met people from all nationalities, groups, couples and singles. Had a great time on it. Would recommend it.