Why does SFDC mean Salesforce?

SFDC stands for Salesforce.com (Sales Force Dot Com)

The company chose SFDC over the simpler SF to avoid confusing with the common acronym for the company's hometown of San Francisco!

SFDC stands for SalesForceDotCom. The company would have retained just SF. But as trends on Social Communities started getting wider the acronym faced the uncertainty of getting hijacked by other trends.

For ex: Initially on Twitter, Salesforce used both #Salesforce and #SF as hashtags to monitor the interactions. Later, it got too crowded to use #SF since it may be SemiFinal of an event etc. So then it decided to switch to SFDC. Now this has gained the prominent Keyword status on many communities(including Stackexchange) and also on Google Rankings.