Muslims claim that Islam is growing, this is due to the fact that they are allowed to marry up to for four wives and breed incessantly. Significant portions of the Islamic world are poor and more often than not poor people are more likely to have more kids than affluent ones.
However, there is another dimension to Islam's growth in numbers relating to conversion. The overwhelming majority of the people that convert to Islam are; the confused, the runaways, the people in prison, the uneducated, etc. Islam preys on the vulnerable and offers a false sense of hope and belonging. While a Muslim intellectual is likely to question Islam and possibly eventually apostate, the oppose is true of those with limited formal education. If Bill Gates and Steve Jobs (and others like them) convert to Islam in substantial numbers, then Islam deserves a second thought.
Also, a disproportionate number of converts leave Islam within two to four years of their initial conversion. Accordingly, one would be hard pressed to find a Muslim convert from a decade or decades ago.
It's okay to believe in things when you're a child (like Santa Clause). However, as an adult, it only brings about laughter at the foolishness of a seemingly "grown" person engaging in such superstitions as a big man living above the clouds who brings rain and other goodies to us because he loves us.