Perceptions (and valuations) have shifted a lot in recent years, and, as the article points out, the "Billion dollar club" is getting crowded.
YouTube was bought for $1.6B after all. That was a huge deal then. When Zuckerberg refused the $1B offer from Yahoo!, that was also a pretty big deal. Now we see a few young companies at these valuations, so, the perception is different.
Much of the new billion dollar club is special in that it actually has the kind of revenue and revenue growth to at least sort of justify the valuation. It sounds like Airbnb, Dropbox, and Square have serious profits with trends suggesting that they'll be even bigger soon. These startups look more like PayPal, which was a 1.5B acquisition where Ebay totally and unsurprisingly got their money's worth, rather than a social site with no revenue. (Did YouTube have any revenue when they were bought? And I think Facebook had ads, but they were still losing money fast when they were offered 1B.)
Even though it's looking like YouTube will pan out for Google, it's a lot more obvious that Square has the potential to be worth billions than that YouTube did.
YouTube was bought for $1.6B after all. That was a huge deal then. When Zuckerberg refused the $1B offer from Yahoo!, that was also a pretty big deal. Now we see a few young companies at these valuations, so, the perception is different.