100 people own 1 share each of a company at $1.00 a share: 100 shares times $1.00 = $100 market cap. Then a new person wants to buy a share from one of the existing owners. The existing owner wants $1.10. Now the stock is worth $1.10 and the value of all 100 shares is $110. The person who sold their 1 share made 10 cents profit, but the remaining $9.90 of new market capitalization was created out of thin air and doesn't exist until those owners are able to sell their shares on the market.

It is possible for actual value to be destroyed or lost to other people.

If you own an oil well and pay out a ton of employees to try a new extraction method which causes a fire - the actual value of your asset has been destroyed. And a lot of money has also been transferred from investors to employees. The embarrassment may also cause you to lose trust of other companies which hurts the value of future contracts. Some of the value you destroyed is gained by other companies since they can now sell their product for more - but the shareholder value of the well you own is lost.

I agree Facebook's value has a speculative element to it - but its pretty clear that the money has been lost because the thing of value has been partially destroyed both by itself and by competitors like Apple and TikTok which is stealing its revenue.

you can equally say - where did it come from?

It went back to where it came from: nowhere. Market cap is a completely made up concept in the first place.

It never existed.

Cash value is realized upon liquidation. Before that, it’s just paper.