How are all these silly web startups raising so much money?

Firstly, I'd like to clear some things away.

1. There is (more or less) consensus that the bubble today isn't in the companies, it's in the Venture Capital (VC) market (see this post, and many like it). There is just more money out there than there are good companies to invest in. Apparently even more so in the Valley, since VC money are clustering there at an accelerated rate.
2. Seed money (which you mention) isn't millions of dollars, and usually not VC money. Seed commes from Angels or Super Angels and are usually sub $1M, commonly around $100K. The funding stages are usually Seed, Early, Growth, (Consolidate), IPO. They are commonly known as Seed, Series A, B, C, consecutively. Where Series A is where the VC enters and it's usually the "Early" stage investment.
3. I would bet you that if you came to Silicon valley investors wouldn't throw money at you.

Raising money is hard and it's an art form.

And be very careful at who you call Silly. I'm sure Facebook seemed very silly before they were where they are today. Also did twitter. And Why, oh why, would someone want to build a search engine when everyone was using Altavista?

But there is a good question here: How do you raise VC money (preferrably at a good valuation).

Here are some things that will help your chances.

1. Build something real. The days of raising money on a business plan are (commonly) long gone.
2. Having done it before. VC like people who've done it before. It's debatable if that makes sense, but I gather they hope they won't make the same mistakes twice. If you would research the companies you'll often see that they have experience from founding other companies, or having worked at VC funded companies. I've got a good friend (ex Google PM) who jokingly claims that "Anyone who's ever worked for Facebook can raise VC money today".
3. Prove that your idea is big. You have to convince the investor that there is a BIG opportunity and they it's within your reach.
4. Prove that you have the experienced team that can both build this thing, and attract the right talent when you start growing and need to expand.

Basically, nail "Idea, Team, Execution", prove that you're unlikely to make common mistakes and attack a large enough problem.

For the VC, it's a numbers game. They plan on making one out of ten good bets. The rest of the companies either fail or don't return their money. The tenth company need to make 10x or more money.