There are always inherent risks to running a startup that's dependent on the resources of another company, especially its APIs.
That was exactly the case of Socedo, a marketing tech startup that recently closed its doors after losing its coveted access to parts of Twitter's API. In a major effort to fight back against abuse, Twitter had decided to restrict third-party access to its API.
(I have some personal identity with Socedo, with memories of the former CEO trolling me on LinkedIn as well as my work email.)
Socedo sold its engineering talent to another startup called Azuqua that offers integrations between cloud-based services such as Slack, Salesforce, and Marketo. It's worth noting that this startup, in and of itself, is dependent on third-party API access.
API access is always voluntary, never an obligation. It can change at any time, and without notice, at the proprietor's sole discretion. If you decide to run a business that lives off of someone else's APIs, do so at your own risk.