Hundreds of thousands of developers know that building apps that rely on the Facebook or Twitter platforms comes at a risk—at any time, the companies can change their access rules or launch a competing feature. The risk is often worth the built-in audience and data from millions of users. Just look at how Zynga grew. Lately, though, as both Twitter and Facebook strain to bring in more revenue and threaten to tighten their platform rules or alter their features to this end, the perennial uncertainty is worsening.