I'll be the first to argue that the internet has ruined everyone's ability to spell, but we can easily overestimate the effect that being online has on our language and presume wrongly that the rate of change today is faster than ever before. Research conducted at the University of Slovenia suggests that word frequency for the most common phrases in English was much more variable 500 years ago than in the present day.
The study by Dr Matjaz Perc examined a large corpus of text, claimed to be 4 per cent of all books ever published since the sixteenth century, to see if they conformed to Zipf's Law. In simple terms, Zipf's law suggests that the most frequent word in a natural language will be twice as common as the second-most frequent, which will in turn be twice as common as the third-most frequent, and so on. The same pattern is seen in many other environments, including how the web has grown and how we acquire new friends.
Zipf aside, the interesting observation from the new research is that these popularity rankings did not stabilise until printed books became extremely common. Tracking the most common words and phrases shows far more deviation in the early years of the printed word:
During the sixteenth and the seventeenth century, popularity was very ﬂeeting. Phrases that were used most frequently in 1520, for example, only intermittently succeeded in re-entering the charts in the later years.
However, that ranking had become very stable by the turn of the twentieth century, with frequent words remaining much more fixed (though not quite absolute). We don't yet have enough data to rank how online usage might change those rankings (if indeed it does). Undoubtedly being online is changing some of our linguistic habits, but relative to the history of written language, we're still in a stable period.
You can check rankings of the popularity of terms over the past four hundred years on Perec's site.
Evolution of the most common English words and phrases over the centuries [Journal Of The Royal Society Interface]