When your service is dependent on another, you're always at the mercy of an update that can shut down the API feature you were using, whether the intention is to kill your service or not.
In this case it seems like the goal of the website is to become a starting point for a person's browsing/searching experience, facebook and google want the same thing. If they feel threatened they could cut just stop Greplin in particular from accessing their API. It's not hard for them to update their API terms of service to make that kind of use forbidden, for instance. That kind of tactic has been used in the past.
That's why I see a lot of uncertainty in the future of that service. I'm not worried about the founder and the company, though, their most likely exit is a talent acquisition by Facebook or Google.