$Shopify(SHOP)$ $Alphabet(GOOG)$ $Alphabet(GOOGL)$ $Pinterest, Inc.(PINS)$A major activist firm just took a big stake. Pinterest hasn't exactly been a stellar performer, with shares more than 75% below their highs. However, Elliott Management, a large activist investment firm, just took a big stake in Pinterest and has reportedly been in talks with the company on unspecified matters over the past few weeks.Here's a rundown of what investors need to know about Elliott Management's investment, why the firm might be willing to put over a billion dollars in capit