Just note, the poster most probably did not actually 'short' facebook, the poster used options to do this. Its pretty simple for the buy case.
Options have a strike price and an expiration date. You buy a CALL option, with a strike price, that gives you an option to buy that stock for that price. Or you can buy a PUT option, that gives you the option to sell at that strike price.
Example using made up numbers. FB is $240. You look on the options tab and you can see various expiration dates. Lets pick 3 months out. You may see a Call option for a strike of $250 for $20. That means that you have until 3/4/2022 to exercise that option, if FB is trading for > $260, you will make a profit. ($240 + $20). If FB doesn't trade you make nothing.
Puts work similarly. The strike would just be $230. Note, you don't have to exercise, if next week FB goes to $300, your call's value will skyrocket to ~$60-70. You can sell the option any time, you don't have to wait.
You may have noticed the cost of the option is greater than the delta between the exercise price and the value of the share. In my example this is roughly due to the time between now and the exercise date. Over time, this shrinks, this is called beta-decay.
