By Nell Mackenzie (Reuters) – Investors have been adding to bets that Credit Suisse’s shares still have further to fall after a social media storm forced a fresh look at the Swiss lender’s problems. A four-fold increase in the amount of the bank’s stock borrowed by investors over the past two weeks reflects a spike in so called “short selling” or “shorting” of the shares. This bet by speculators, such as hedge funds, comes after a number of tumultuous weeks for the bank and underscores the scale of the challenge as it prepares a structural overhaul to draw a line under a series of scandals and…