The semicolon has an interesting history, which is traced rather well by Paul Collins in his 2008 Slate article. I thought I'd go back and have another look at some of the numbers, which can now be calculated with somewhat better reliability.
This graph demonstrates the rise and fall of the semicolon in English. Its popularity peaked right around 1800 at just shy of 1% of the words. That is to say there were almost 10,000 semicolons per million words. That's a little more common than is is today.
|Frequency of semicolon use (Source: Google Books Ngram viewer)|