Because they were pretty big acquisitions, built at a time when building to get acquired by bigger internet companies was a completely viable business model.
At the time, eyeballs had a premium dollar value. Being 2 for 2, Mark is comfortably rich. Clearly in today's market one would need a more concrete revenue model than eyeballs.
Both these businesses generated a large profit for their founders. Maybe if you learned from guys like Mark, as opposed to the IRS, you'd make a better Hacker News commenter and... I daresay, a better entrepreneur?
http://www.irs.gov/newsroom/article/0,,id=169490,00.html