If any of you are in the US and nervous about handing over your SSN, remember that Fiverr:
Is a publicly-traded company worth 5 billion dollars and one that is actively on the New York Stock Exchange and thus subject to a LOT of regulations and oversight in terms of how they handle data, accounts, financials, etc. They’re not going to risk that valuation playing games with SSNs.
Every job you’ve ever applied to, even as a teen - gas stations, restaurants, retail stores, etc - has either a digital or physical copy of your SSN floating around. Doubly so if you actually worked for them, that info is on the W-9 you filled out. I guarantee you the security on those little applications torn off a pad they keep under the counter have FAR less security than Fiverr does, and yet you’ve never worried about those.
You can always go on the IRS website, apply as a sole proprietor, and get an EIN (employer identification number). This functions as an SSN would, except it’s for your “business,” allowing you to satisfy the ID requirement without revealing your personal SSN, if that’s important to you.