Why would they do that? That's what the APIs are for.
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.
> 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.
This is your only 'why', but it's very weak. Every company wants to be the 'starting point for a persons browsing experience', but Facebook doesn't provide that, and neither does Greplin. Google does of course, but Greplin doesn't.
Your point reduces to "companies which consume APIs might compete with the companies which create them, and so might be shut down". Of course, your statement is true, but it's valueless.
Exporting the private data from Facebook is considered a big no-no, from what I remember the TOS might only allow you to do so for a short period. Maybe that search service gets away with it because it stores an index based on the private data and not the private data itself. If Facebook gets uneasy with how close the index is to the original data or what private information leak might happen if Greplin is compromised, that could be a good enough reason to put a stop to it. Facebook not getting into a private information leakage scandal is probably more important than allowing a startup to live on Facebook data.
The problem here is that you have no visibility into Facebook's roadmap, priorities, etc. Sorry, but I think betting $5m (or years of your life) on a startup that can literally be killed overnight on the whim of someone at another tech company is crazy.
In general, we're young in the space of companies that rely on APIs and other companies (App Store much?). Zynga has made a pile of cash relying on facebook, and plenty more will too, and that's just the way the world is from now on.
As for the whim of some individual, you are absolutely right. But that will sort itself out in the next few years, and it doesn't seem like a good reason not to get in early on what is obviously a trend that's here to stay.
It's pretty obvious that a business that's completely dependent upon FaceBook isn't in a very good position. It's equally obvious that a business that's completely dependent upon FaceBook but has users is in a better position than a business that nobody cares about. You start with the latter, not the former.
If Greplin or some other startup is still dependent upon FaceBook after years gone by and $5M spent, they're doing something wrong. But in the near-term, they're doing a helluva lot better than the average startup.