I'm assuming you're being sarcastic -- but the "everyone tweets ten Tweets per hour" figure sounds like it's off by at least one (if not 2 or 3) orders of magnitude.
1) there are far fewwer than 175m active users of twitter
2) a tweet compresses to far less than 140 bytes. Text normally compresses to (easily) a third its size
3) Wikipedia says there are some 300 million tweets per day...or 13 gigabytes per day (compressed).
A single 1 TB hard-drive is almost certainly good enough to store a couple of months of tweets. A year of Tweets should fit on a couple of thousand dollasr of hardware.
and 4) I added the Amazon line so that you guys would know I was being absolutely, 100% sarcastic in every way possible. I guess the clue still wasn't enough.
My point is that there are very few things that are as easy to scale as a platform built on sharing 0.13 kilobyte tweets (0.04 KB compressed), in an age where rendering many common front pages requires a browser to download 600-800KB in static and dynamic content. Come on.
Sometimes the Internet gets sarcasm.... and sometimes we fail badly.
600-800KB +++
I was chatting with my brother in law recently about his website (fashion photographer). His homepage was about 5.6MB. When I asked him about it -- he figured it was normal. Sure enough -- he rattled off a few names for me to check. All of them had front-pages >6MB.
You're forgetting about the network effect involved in scaling something like twitter which is commonly overlooked by people judging services like that.
