Did you know that there's actually an English word that you can use instead of saying "internet users" or "people online"? That word is internaut, a portmanteau of "Internet" and "astronaut" which the Oxford Dictionaries define as "a user of the Internet, especially a habitual or skilled one".
However, despite the fact that this 1990s creation would be very handy in everyday use, it seems not to have gained popularity in the English language. In fact, I've never heard anyone use the word internaut in English, be it in writing or in spoken language. I only learned of its existence today because I was thinking about the Spanish word internauta, which I recently encountered in a translation.
I first learned the word internauta when I studied abroad in Spain in college. Despite never having seen the word before, it was immediately obvious what it meant, and I remember wondering at the time why we didn't have an English equivalent. It wasn't an obscure word either, as it was frequently used on television to encourage people to visit a program's website, vote for a performer in a competition, and do any number of other online activities. It also wasn't uncommon to hear a TV host say that they were going to answer questions from internautas throughout the show.
When I recently encountered the word again in a translation, I ended up just using "internet users", but I wished there was a better equivalent in English. While internaut may have made it into the dictionary, I definitely don't think it's used often enough to make it an acceptable choice in most translations.
One alternative would be the slang word surfer, although it doesn't seem to be nearly as popular as it once was. There's also the added problem that the word's standard definition refers to someone who surfs waves, not the internet.
In any case, if you ever need to say "internet users" in Spanish or Portuguese, keep in mind that you can always use the word internautas! A few other Romance languages use equivalents of this word as well, including internautes in French and Catalan, and internauti in Italian.