I think the killer app is out there waiting. Something like an expert system that doesn't rely on complexity but undiscovered gesture of simplicity.
I liked the idea of Digg, but there are key flaws with its practice. Instead of it being crowd sourced, it's a bunch of pages loved by friends of Kevin Rose. It not so much the wisdom of the crowds: it's a bunch of demented shut-ins. I know a woman who put in a link and got back, "shut the fuck up, you ugly bitch." and similar.
Facebook doesn't have the same potential for enmity because it's a social network: your friends join you; almost everyone reveals their contact information; and you are with birds of a feather-- if you're friends with jerks, they'll be familiar jerks. You can shut them out with simple unfriending.
Twitter. I resisted joining Twitter until recently. I thought that the moniker of "twits" was apt. Apt! Sure Brevity=Wit, but 140 characters seemed to reward people with ADD. Contrast it with Digg: lots of threads, a clique of people who get to the front page, lots of shut-ins who vent at anyone. With Twitter, you have your own posts. People can follow you. You can follow them. You can block people. You can keep your posts locked off from the public at large. If someone wants to spout bile at you, they need to do it on their own dime-- their own twitter feed. Try to find points of contact into the big web ventures: Google, Facebook, eBay and Paypal. That lack of functionality seems to be a boon for them. The lack of complexity in Twitter is part of its strength.
I dabbled with Robinhood Fund. I like their weighting system. People opinions are amplified by the qualty of their participation. People who are jerks get their voiced tuned down. People who are iffusive also get toned down-- you can't like everything. It means the "good" players get the loudest voices. Also, just as data rots, their participation has to be fresh or it rots-- diminishes.
What's so good about Twitter may be coming to an end. Mark Trammell from Digg is moving over to Twitter. If the same opportunities for sniping and toxic anonymity come into play, Twitter will be worse than a billion dollar business with no revenue: it'll be Digg.