Short sellers raised their bets against Tesla this month, as the electric car maker's shares fell by a further 35% . More than half a billion dollars worth of shares were sold short in December, making Tesla the second-most shorted stock as of Dec. 21, according to data from S3 Partners. Short sellers profit when stocks fall. They borrow shares to sell them immediately with a plan to repurchase them when the price is lower to pocket the difference. "Over the last month we saw a resurgence of short selling after a slight lull in November with 3.78 million new shares shorted, worth $520 million," said the note from Ihor Dusaniwsky, managing director of predictive analytics at S3 Partners. Shares in Tesla have declined by 65% this year and 35% so far in December, pushing the company's market cap below $500 billion for the first time since the pandemic. Tesla was worth more than $1 trillion in October last year. More than 3% of all Tesla shares currently trading are sold short, representing $11.3 billion in total short interest, according to S3 Partners. Only Apple faces a bigger short bet of $16.8 billion. Microsoft , Visa and Alibaba 's U.S.-listed shares round off S3's list of the top 5 most shorted stocks. Twitter bid Short-sellers upped their bets against targeting the electric automaker this year after its CEO Elon Musk announced his big to buy Twitter on April 14. Hedge funds expected Musk to sell shares to fund his $44 billion leveraged buyout of the social media giant. Long-time Tesla bull and investor Ross Gerber told CNBC's "Squawk Box Asia" that he trusts Musk not to sell shares in the future, but added that it was right for Musk to sell in the past to fund Twitter. "It's an unfortunate situation but he's running his finances conservatively, as he should," said Gerber, chief executive of wealth manager Gerber Kawasaki. He said he had spoken to Musk directly through Twitter "Spaces." "I've trusted him with a lot of money, and he's done really well for me over a very long period of time," Gerber added.