The answer to this question is open to debate and is highly dependent on how popularity is defined.
The International Handball Federation (IHF) Marketing Webpage lists 209 member federations representing 28,000 clubs, 128,000 teams and a total of 27 million players. These numbers have to be taken into context, however, as some of these nations have negligible handball programs and what exactly constitutes a team in the 128,000 referenced teams is not defined.
For many years the often quoted statement was that Handball was the 2nd most popular sport in the world. This was always a bit of a stretch in light of basketball’s increasing popularity, particularly in China. While #2 may not be true world-wide, handball is the consensus #2 sport in several countries. This FAQ contains maps depicting where the sport is popular on each continent: Link