If someone had told me in 2001 there was a way to easily annote webpages and share them with my friends, I'd have yawned. BORING!
Some concepts are just boring, and technology improvement does not change it. Those companies failed because people just don't have a need to mark pages. I can just copy and paste the text into emails.
Technology doesn't just change what's possible, it changes what people want. The reason there's demand for something like Twitter is because now everyone is online, nearly 24/7, and so the value of an online service that tells you what your friends are doing has gone up significantly.
You may be right - actually, from a numbers perspective, you probably are. But if I had a few million to invest and the chance to blow $15K on something like AwesomeHighlighter, I'd think there was enough of a possibility of success to make it worthwhile. People are reading more online, and their reading is much more participatory. That's a different environment than in 99/00.
Create a short url for a page - AND mark the parts you find interesting. Tiny url but with a marker.
The fact that others have tried the same idea and failed is probably a bad indication, but it's not a hard rule that this attempt will fail.