They're not really comparable when you look at the numbers. Facebook had 500 million active users, a steadily growing user base (1 billion was projected within a year at the time of the IPO), and a clear monetisation strategy.

Snap had 150 million active users, most importantly it's growth was seriously stunted, and it's monetisation strategy was rather 2 dimensional.

Facebook IPO'd at around $100 billion, Snap IPO'd at $20 billion, and it got very close to $30 billion in the trading immediately after. However nothing about it's numbers should justify that. Investors get seduced by the promise of a SaaS revenue hockey stick, and I hope people learn the lessons of Snap before they buy into future hype IPOs so easily.