if you have even spent 10min researching Buffet's investment philosophy, you would know he doesn't invest for speculation.
He wants companies that produce a needed good or service that involves a revenue. And they have to be at a valuation that is a good deal. Twitter currently produces no discernible revenue, so would be completely counter to Buffet's philosophy .
That's doesn't mean these internet companies won't pay off; but if you want to invest in such stocks or fund then BRK isn't the place to be for it.