The popular ride-sharing application Uber is looking to raise another $2 billion in private funding. According to reports, this would value the startup at $50 billion. If Uber can obtain these funds, it would make Uber the highest-valued private startup of all time. The only other privately owned startup to achieve such a high valuation before issuing an IPO was Facebook (FB), and it is now valued at $226 billion.

Obviously, Uber’s valuation is exploding; it continues to grow at an astronomic rate. As recent as February 2014, Uber was valued at roughly $18 billion. The company is already the most funded startup of all time, having raised $5.9 billion in 6 years. Yet Uber’s success has not come without many problems. It continues to fight multiple lawsuits, including claims that Travis Kalanick stole the initial ride-sharing idea from Kevin Halpern, allegations of unlawful business practices, and antitrust assertions; it also faces increasingly strong competition from Lyft.

Furthermore, smaller Uber competitors, like Sidecar and Hailo, are gaining popularity. Egregious examples of “surge pricing,” during times of high demand, have made some customers resort to these cheaper alternatives. For example, San Francisco’s popular music festival Outside Lands featured Uber rates that were 5x greater than normal. This resulted in festival attendees spending more on their rides home than their actual concert tickets.

Uber’s $50 billion valuation continues to ignite speculative debate about whether or not the Silicon Valley is a private equity “bubble.” In 2014, Uber generated $400 million in revenue. For comparison, when Facebook was valued at $50 billion it was generating annual revenues in excess of $2 billion. Not surprisingly, most of Uber’s valuation is based on potential growth rather than current cash flows.