It's privately owned, but by a number of parties all with a lot of money on the line. That person may have pulled together the investors, but it most definitely has investors that it needs to answer to in the medium term. In the long term, they have stated that re-floating is a goal.

The valuation of the company matters to the banks and VCs who own X.

And the primary way to evaluate the worth of a social network is user engagement metrics.