Twitter has a very clear business model: advertising. They are on track to make $150+ million this year from it.
If you want a proper comparison, Google's PE ratio was 150 at IPO (and 300 at the high range of estimates, and it eventually reached that)
A lot of smart people thought that was a bad buy at the time, as well:
I would suggest that Twitter are likely waiting to book an entire year of profits and 3-4 years of revenue growth before they file