I searched the internet for reliable data on a list of languages by total number of speakers, and I ran into a dead end. It seems like every site on the internet claiming to have an accurate list all stem from this one source: http://www.ethnologue.com/
I searched through the bibliography for any current studies that support this, and most of it apparently comes from outdated census data and guesswork.
The reason I was looking for this in the first place was to solidify claims that Mandarin Chinese was the world's most populous language, however, all I could find was the happenstance claims from random sites that all link back to Ethnologue. I can understand that it might be spoken by the largest number of native speakers, but that is really a moot point when what I really want to know is how many people speak each language, regardless of whether it's their first, second, third, or seventeenth language.
This paper interested me, because instead of giving raw facts without any sort of historical background, he synthesized the data into possibly trends. At this point though, I am equally doubtful of this information, since I'm unsure as to whether or not it has been peer-reviewed.
I seriously doubt that there are a huge influx of immigrants to China, and since Mandarin is not really a lingua franca, despite the emigration of many native speakers, I have doubts as to the influence of Chinese on the rest of the world. Especially since the trend has been thus far for other countries to learn English, while English speakers continue to propagate English. For an example of that, I would use this diagram. However, I am dubious as to my own bias, and the paper that published that study.
It would have been fascinating to find hard facts to illustrate the status of various languages in the world, however, at least for the moment, I haven't found anything rigid enough to support any valid conclusions.