[hayksaakian]

would it be in their shareholders interest to pay much more than the market value of their company?

if nobody else would be willing to pay half as much, isn't someone to blame for overpaying? are there any consequences for this?

[tyre]

That depends on the founders' willingness to sell.

What the market will pay isn't equal to the price at which the owners will sell. It may have taken a large amount for them to sell at all, with Twitter being the only one to whom such a price was "worth it."