You almost certainly already are, and mostly that is fine. The question is whether you have priced the dependency honestly.
Read the terms of service, actually. Not for legal comfort but for the operational facts: can they change pricing without notice, can they terminate for convenience, do they forbid your use case, do they claim rights over what you build, what notice do they commit to before a breaking change or a shutdown.
Then assess concentration. A dependency you could replace in a fortnight is a purchase. A dependency whose removal ends your product is an existential risk with someone else’s hand on the switch. For anything in the second category, know the alternative, keep your integration behind an abstraction you control, and keep your own copy of the data you would need to migrate.
Watch the leading indicators too: a platform that stops shipping, stops answering support, starts building first-party products that look like its ecosystem, or gets acquired is telling you something before the announcement arrives.
None of this is an argument against building on APIs. It is an argument against the belief — which I have watched break a lot of businesses — that an API you depend on will always be there. Parse taught a lot of people that lesson at once, and the lesson keeps needing to be retaught.
