Having more than 100 million daily active users was the easy part for Snapchat, now it needs to find out how to make money off them. The company has tried a number of approaches, most of which involve various forms of advertising, and some of them show promise, but Snapchat’s ad revenue is still pretty lackluster, and investors are worried. Concerns that Snapchat’s $16 billion valuation have been circulating for a while, but the company’s inability to make an impressive amount of money from ads is making things much worse.