My guess is its the exact same proportion as offline. Meaning only a small percentage make their living off of the internet as a business and most go belly-up as is true in the general marketplace.
The only difference would be probably many more are attempting to earn a living off the internet than offline and the ability to earn a couple extra bucks is much greater than ever before.
With that said, the internet offers great potential to earn a living but very few attain it. Most make a few bucks here and there. Maybe 100 to 500 extra dollars per month. Some are in the 500 to 1000 range. 99% of ebayers are in this category.
Off eBay and Amazon, my guess is 99 out of 100 websites fail after the first year or break even.
From my readings, there are three ways to make money online. Offering a product or service, SEO (Search Engine Optimization) and affiliate marketing with affiliate marketing requiring some SEO knowledge.
Personally, I have only ever been involved with offering products for sale and have had some periods of great success and periods of little success.
I have only ever sold on eBay with a couple of Craigslist and other site sales.
My greatest successes came from jumping on the bandwagon in certain computer parts a couple years ago; selling for a collectibles shop on a commission basis; and finding stuff to sell.
My first eBay sale was finding a Cricket bat at a Goodwill store for 7$ and reselling on eBay for 25$.
I should add that I did pretty well drop shipping watches from a supplier in Asia until eBay's new DSR rules.