This article is from October 2007, but it has some interesting quotes from Niamh Ni Mhir at Hostelworld:
"While we have seen steady growth in all our top destinations such as London, Dublin, Barcelona and New York, other destinations such as Dubrovnik, Hong Kong, Seville, Grenada and Athens are giving them a run for their money.
Further afield, South America showed the largest percentage growth this summer, with Peru coming out as the backpackers’ favourite destination. Asia also saw continued growth with booking numbers at more than twice last year’s, in particular into Vietnam and China. With the impending Beijing Olympics in 2008, we are expecting further growth in China. We foresaw the potential of the Asian market in 2006 and set up an office in Shanghai to capitalise on the growing popularity among travellers to Asia.
The article says the weak dollar was not having an impact on bookings from Americans. Americans, English, and Australians are their top bookers. It also mentions an increase in bookings from Brazilians and South Koreans.