I don't think the big microjob platforms publish exact monthly figures, but those that are publicly traded (like Fiverr or Upwork) probably move tens of millions of dollars a month. Although margins are tight due to marketing expenses (mainly) and support.
The main challenges, to name a few, are balancing supply and demand (users and sellers), controlling fraud and payments, and retaining users so they don't do work outside the platforms.
I don't even want to think about those figures; they must be incredibly high.
Another thing is that many people overlook these platforms. I think they don't just grow by "having users"; they need a solid monetization model. Most of them depend on low commissions that only work when there's a huge volume, which makes it difficult for a new project to generate profits quickly (as many of us hope).
