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Facebook Inc. (NASDAQ: FB) reached an extraordinary milestone as its market cap rose above $500 billion, and it closed at over $499 billion on Friday. It is now worth more than Amazon.com Inc. (NASDAQ: AMZN), which has a market cap of $488 billion. Unlike most other companies in the top tier by market cap, Facebook is only a little more than a decade old.

Facebook moved ahead of Amazon because Wall Street was impressed by its earnings while Amazon’s were disappointing. Facebook’s revenue increased 41% in the second quarter to $9.2 billion. Net income was $3.9 billion, up 71% from the same period a year earlier. Its profit margins are breathtaking. Amazon is by far the larger company. In its most recent quarter, revenue rose 25% to $38 billion. However, its net income was only $197 million, down $857 million from the same quarter a year ago. Its core e-commerce businesses lost money. The bottom line was rescued by its cloud business, known as Amazon Web Services (AWS).

It is hard to make the case that Amazon is anything other than a wild success. That does not keep Wall Street from its anxiety about its bottom line. Facebook does not face similar criticism.

If market cap is anything, it is a measure of investor voting about a company’s future prospects. Facebook has no competition. It has 1.3 billion members worldwide. Its revenue growth rate will be over 50% for the next several quarters. There is a case to be made that its revenue will hit $100 billion in less than three years. Its profits at that point could be $30 billion. That would make it one of the half-dozen most profitable companies in the world.

Amazon proponents make the case that one of its advantages is its extraordinary innovation. Its Prime membership services have created a bond of loyalty with millions of customers. It is ready to deliver packages with drones. Amazon’s buyout of Whole Foods Market puts it squarely into the huge U.S. grocery market. And, although AWS has little relationship with the company’s core e-commerce business, it is the leader in the cloud sector. Facebook does not have a second promising business. However, investors obviously believe its single core business is enough to create extraordinary value.

Second-quarter earnings were a watershed for both Amazon and Facebook. Each dominates its industry. The issue is how keeping that dominance costs each of them on the bottom line.