It’s Not a Bubble, People; It’s a Pyramid Scheme 
Mark Cuban's statement is only true if the VC financings are simply a buyout of previous investors.  Many of Facebook's recent financings were capital injections + buyouts of founder and early VC shares. I don't believe this is the norm.  Simply, VCs at round t-1 typically participate in round s > t and if they don't, future investors will not buy them out.  If they did, it would simply mean that none of the money going to firms like Twitter and Zynga is used for investment.