Values each share of Twitter stock at $17, yet GSVC carries their 1,835,600 common shares and 65,000 Preferred shares on the books at $36.1M, or $19/share, so it seems like this announcement should make them have to lower the value of their stake.
At least GSVC purchased their stake at avg $17.36/share, in the same neighborhood as BlackRock. One of GSVC's most common criticisms is that they overpaid for all of their holdings. While still true for their tiny stakes in ZNGA and GRPN, at this point they would turn a small profit if they sold their shares of FB.
It all comes down to whether you believe GSVC has overpaid or underpaid for these private companies. Obviously the market sentiment is still they've overpaid, explaining the huge discount to the stated NAV.