The Wikipedia page, Languages user on the Internet provides two different ways of ranking the most popular languages on the Internet.
Estimated percentages of the top 10 million websites using various content languages as of 4 March 2017
Under this list, Arabic appears 16th, at 0.8%.
Estimates of the number of Internet users by language as of June 30, 2016:
Under this list, Arabic appears 4th, at 4.7%.
The source for the latter statistic is Internet World Stats.
It estimates there are 173,538,690 Arabic Internet users - 42.5% of the Arabic population.
I find this source doubtful: it implies that almost half of Arabic speakers (including children) have Internet access and that they use the Arabic "language" (let's assume they all use Modern Standard Arabic).
How do they collect such stats? Is this accurate that there are that many Arabic speakers using the Internet and yet producing a disproportionately low percentage of the content?