Fair enough. The standard growth adjustment to the P/E ratio is the price/earnings to growth (PEG) ratio. Let's consider a price/revenue to growth (PRG) ratio for Facebook and Snap.
Facebook's $398bn valuation [1] sits on $27.6bn of 2016 revenues, up 54.2% from 2015 [2]. This represents a PRG of 0.266. Snap's $30.4bn valuation [3] rests on $404 million of 2016 revenues, up 690% from 2015 [4]. Thus, a PRG of 0.109.
Snap needs to grow revenues by at least 290% a year every year for the foreseeable future to reach Facebook's price/revenue levels. Put another way, Snap has a year and a half [5] to hit $2bn in revenues. (At that point, its price-today-to-revenues-tomorrow profile will be similar to Facebook's price-today-to-revenues-today.)
[1] https://www.google.com/finance?q=NASDAQ%3AFB&ei=z_C9WLngO4W8...
[2] https://www.google.com/finance?q=NASDAQ%3AFB&fstype=ii&ei=JP...}
[3] https://www.google.com/finance?q=NYSE%3ASNAP&sq=snap&sp=2&ei...
[4] https://www.google.com/finance?q=NYSE%3ASNAP&fstype=ii&ei=1v...
[5] log 1.29 ((27.64 / 398.3) * 30.44) / 0.4045)
Disclaimer: this is not investment advice. Please don't be a numpty and buy or sell securities based on Internet comments.