PhD (at Oxford) vs staying at in the industry (Facebook) for a ML student?
You must make this decision - no one else can make it for you. Things to think about:


Uninteresting jobs are bad. If you can't see your job getting more interesting, then say 5-10 years down the line there's a good chance you'll get so bored your performance starts suffering, and then you might get fired.
You know the industry very well since you have a first-hand insider view. What jobs do you enable by getting a PhD? If they're more interesting jobs (do you actually know they're more interesting, or do you just think they're more interesting?) that you will never be able to do without a PhD, that's a powerful reason to get a PhD. On the other hand, it's conceivable that you'll be able to do those jobs without a PhD simply by staying put, acquiring more experience, and doing well. You'll know better than anyone else. 
Do you have a significant other or a family to provide for? If so, what do they think? Remember that usually, once people experience a luxury, going back to a less-luxurious lifestyle is typically a big no-no even if it was previously acceptable. Even if you can accept it, your significant other / dependents might not.
What does your current manager or other senior colleague think (if you trust them enough to talk to them)? Just as important, are they willing to rehire you after you graduate?
Is it actually economically better to do a PhD? Run some calculations. If you stay put, you earn $X this year, $X + [increment] next year (if you have one), etc. If you go to Oxford, you earn $Y for 3 years where Y &lt; X, but then after 3 years you earn $Z where Z > X. How many years does it take before the second option exceeds the first, if it exceeds the first at all? The fewer years this takes, the more attractive the PhD becomes.


These things are too intricate to say more, unfortunately. As above, you must make the decision - no one else can make it for you.