Twitter shareholders are trying to decide if they should cash out, All Things Digital's Kara Swisher reports.

Russia's DST Global has reached an agreement with Twitter to buy as much as $400 million worth of stock from investors and Twitter employees.

They can sell 20% of their stakes at an $8.4 billion valuation.

There is, however, a cautionary Valley tale for anyone considering the sale route.

Two years ago Facebook shareholders were faced with a similar deal. Those who sold when (who else?) DST invested $100 million have since seen their former shares skyrocket 10x in value.

Whoops!