Upwork makes money from monthly subscription fees, from a percentage of revenue on each project, from miscellaneous fees such as promoting a job and buying the option of inviting extra freelancers, and from sale of connects. As UW has just recently rolled out the charge-for-connects revnue stream, it is too early to know which stream contributes the highest revenue or the highest margin.
Percentage of revenue from each projec is my guess as the most lucrative area. I do not find the 20% fee because it is lower than my businesses' overhead to produce leads, manage payments, and the rest of the services UW provides.
Internet job boards were all started by engineers and/or code-writers, with a few exceptions. UW has high costs because of its size. I occasionally hire on UW and I used to do freelancing before my health led me to retire. I understand that UW has not yet made a profit; it's entirely possible that the entire industry will never make a profit as a whole.
I think Bill's answer is excellent.
It has been reported in various places that Upwork has "never made a profit."
I have no reason to dispute that.
But to my simple mind, I'm not sure exactly what that means, or how meaningful it is.
From what I have seen, there are many companies which have never made a profit, but which are very successful, and which have very wealthy shareholders, CEOs, and well-paid employees.
Think of those movies you hear about, which made hundreds of millions of dollars at the box office, which paid their stars salaries in the multiple-million dollar range, paid all of the people involved attractive salaries, paid the screenwriter and director a few million each, yet the movie "never made a profit."
I know nothing about economics, so I can't explain all of that. But when I look at the fact that Upwork and its predecessor companies have been around since 1999... And when I look at the fact that Hollywood keeps making big-budget movies even though none of them "make a profit", I just figure that these things are not something I should worry about personally.