Zynga is worth $7.5 billion? SecondLife’s valuation is flat? SharesPost dishes the dirt.

SharesPost, an online marketplace for buyers and sellers of private company stock, today dropped a set of research reports on some of the companies of interest to its users. The most notable was social gaming startup Zynga, while others included virtual world operator Linden Lab (aka SecondLife) and commercial rooftop solar cell maker Solyndra.

Each of the reports is more than 50 pages long, and can be downloaded here (free, but you need to register first). Most of the financial data is estimated, including market capitalizations. But it’s still fun to browse:

Zynga: Estimated market cap of $7.56 billion, compared to $5.22 billion last November. This one is particularly interesting, given a report that the company is finalizing a new VC round at a $10 billion valuation. Guess that means NextUp Research, which authored the reports for SharesPost, doesn’t believe quite all the hype…

Linden Lab: Estimated market cap for the social widget maker is $367 million, which is up just a tick from last November. The company last raised VC funding in 2006 at a $141 million post-money valuation.

RockYou: Estimated market cap of $230 million, down substantially from $345 million last November. The blame goes to a 50% decline in the company’s Facebook user base. It last raised VC funding in June 2010 at a post-money valuation of nearly $400 million.

BrightSource: Estimated market cap of $868 million, up 28% from last August. Credit goes to new cash on hand from a $90 million VC funding round and expectations of increased solar adoption.

eSolar: Estimated market cap is $707 million, up 12% from last October. Again, this is based on an optimistic belief in solar’s future adoption. eSolar last raised VC funding at a $665 million post-money valuation.

Solyndra: Estimated market cap of more than $1.6 billion, which is a slight decline from the last time around. This is a bit bizarre, given that Solyndra’s early VC backers recently agreed to convert preferred shares into common, and last year considered raising money at a $250 million valuation.