Do you think twitter has such a huge population now that another startup along the same lines does not have a chance of succeeding ?
But, there's still the problem of Twitter. There are so many Twitter based apps out there, that spread their brand which means there's about 6 gazillion sites other than blogs, and now news sources like CNN, that actually promote the use of Twitter. Twitter is big.
In other words, maybe the best way for someone to take down twitter is to create a compelling reason to not use facebook's status feature, and instead use the next new thing.