This TvWiki site doesn't date its info, but it does make this statement:
"Jehovah's Witnesses have a small active presence in most countries and are the second or third largest religious group in many countries with a dominant religion. In no country are they a large part of the population, however. Brazil and Mexico are the only countries other than the U.S. where the number of active Witness publishers exceeds 0.5 million. The highest proportion of Witnesses in a country of substantial size is in Zambia, where 1% are active Witness publishers. Growth in most developed countries is slow or negative in recent years but is offset by rapid growth in less developed lands, particularly the former Communist bloc and Latin America. .."
From this link:
The information seems dated, however, since U.S. 'growth' has generally flatlined in recent years....