Twitter is now worth only one third of what Elon Musk paid for the social media platform, according to Fidelity, which recently marked down the value of its stake in the company.
Mr Musk, who has acknowledged he paid too much for Twitter, offered employees new equity grants this year that valued the company at $20 billion.
It is unclear how Fidelity arrived at its new valuation or whether it receives any non-public information from the company.
Fidelity first reduced the value of its Twitter stake in November, to 44 per cent of the purchase price, which was followed by further markdowns in December and February.
Twitter has struggled financially since Mr Musk took over.
After saddling the company with $13 billion of debt, his erratic decision making and challenges with content moderation led advertising revenue to decline by 50 per cent, he said in March.
An attempt to recoup that by selling Twitter Blue subscriptions has so far failed to take off. At the end of March, fewer than 1 per cent of Twitter’s monthly users had signed up.
Mr Musk’s investment in Twitter is now worth $8.8 billion, according to the Bloomberg Billionaires Index, which uses Fidelity’s valuation to calculate the worth of his holding.
He spent more than $25 billion to acquire an estimated 79 per cent stake in the company last year.
The latest markdown erases about $850 million from Mr Musk’s $187 billion fortune, according to the index.
Despite Twitter’s issues, his wealth is up more than $48 billion this year, largely because of a 63 per cent surge in Tesla’s share price.