How do you know that Chrome and Safari are supported? Is there some documentation of this? A list somewhere? If so, Facebook should at least link to that page from here.
Interestingly, that page doesn't mention Opera.
Although interestingly, if Opera accounts for ~2% of FB users (big if), then FB would have 16MM Opera users. Using the average of $4/user/year, that means Opera generates $64MM for them. That would be more than enough to justify throwing a few devs at.
Edit: Also curious is the pointless URL that page has. I would have thought FB would be using basic SEO on their help pages, so that someone googling "facebook supported browsers" would have a better chance at ranking first (it does for me, anyway). As an aside, the sub's page is actually the second result.
Safari I don't know, are FB obligated to inform people of every possible choise? Nope, they aren't. Now let's discuss something more interesting than this dull facebook page!
You know that, but there's no guarantee whatsoever that someone landing on this page knows that.
It's in FB's best interest to inform people of what works and what doesn't. They certainly don't test every single browser out there, so putting the browsers they test on onto this page keeps users on the happy path.
It isn't malicious, it isn't "stupid." It's "corporate." ("We'll fix it later.")