Wow, a $250 valuation with 6 million users? That's $41.66 per user. It is ridiculously high, especially with no business model in sight.
I honestly think that Twitter could become The Next Big Thing(TM), so I don't think the investors are crazy for investing at a high valuation.
Which is why they don't have a competitor. It still looks like a crappy businesses that's at the mercy of the wireless industry. EX: If they could make money from SMS traffic face book would take about a month before they had a system like this running.