This link shows the top 10 internet languages:http://www.internetworldstats.com/stats7.htm
According to this, Arabic is at number 7 with 65.4 million users. French is closely behind with 59.8 million users.
But what is most significant is the growth rate in internet users, of the top 10 languages, Arabic has had the highest growth in internet use (2,500%, considerably higher than the French growth rate of 398%). If those growth rates were to continue then Arabic will be growing into the top 5 or even top 3 languages of the internet within the next few years.
I suspect the reason why Arabic has more internet users than other languages of the Islamic world (i.e. Turkish, Urdu and Indonesian) is not the number of Arabic speakers, but rather the wealth of oil rich Arabic speaking countries.