Islam and Christianity are more like organized business cartels, who hunt for new converts through various (sometimes nefarious) means. Let's compare converts to Buddhism to those to Islam. Converts to Buddhism mostly comes from the general population, and are overwhelmingly represented among the white-collar population. On the other hand converts to Islam mostly comes from the blue-collar population, and most of the times it is the ethnic minorities who convert to Islam (African-Americans, Hispanics.etc). Also a large part of the conversions to Islam occur in prisons.
You are right when you say that Islam is successful in converting a large number of people from other religions every year. But in my opinion, the quality matters, not the quantity.