As a reference, I run https://doarama.com and looking for comparisons.
Revenue is easily above the low 7 digits, Net is above the low 7 digits. Essentially the ceiling is not artificially limited, your abilities though are.
I purposefully didn't select a metric. Was going much more for what the community judged. Could be users, rev, scale of impact (however that would be defined).
I'll have to look at the stats on each of the responses to see what kind of numbers craigslist, and pof had while the founders were still going it solo.
Your ability to produce is limited by the number of hours you can work. The revenue and income is limited by whom you are selling to. The more clients you sell too the less likely you can scale before taking on staff. So typically you'd have fewer clients at higher revenue per client. Which also has its own issues.
In general, and in my opinion you wind up hiring staff for almost any business once you cross $500k/yr run rate or you have more then 5-10 active clients/projects that require weekly touches. The level and details of what that staff is and the cost varies greatly.
That said there are solo founders generating more.