Just in from a source involved with the transaction – the $100 million Facebook Employee share buyback has been oversubscribed (its not clear by how much), and ex-employees are being cut back significantly.

The program was announced on July 13. Up to $100 million in employee common stock was to be purchased by Russian investment group Digital Sky Technologies, which also funded a separate $200 million venture investment in May 2009. Employees, ex-employees and other common stockholders could participate, and had to indicate their intention to sell by early August.

People were limited to selling up to the lesser of 25% of their total holdings or $1 million. Shares are being purchased based on a $6.5 billion valuation.

Employees were given first dibs on stock sales, we’ve confirmed. Ex-employees were notified up front that they’d be able to sell only after current employees. And it turns out it made a difference – the ex-employees were cut back to “a small fraction” of the original amount requested. Some executives and larger shareholders may be able to sell additional stock via a separate arrangement, says our source.

Facebook declined to comment on this post.