If you look at all the value that Facebook (and many other companies) have derived from working with PHP, it's definitely a "because of PHP's strengths" and "in spite of PHP's problems".
No technology is perfect, but imperfect technology that lets you Get Stuff Done(tm) goes a long way.
What strengths of PHP are you referring to here? TCB (Taking Care of Business) seems to be a programmer trait. Considering they have written their own implementation of PHP it seems like PHP didn't offer them much other than then original authors knew how to write it. I could be talking about any language in that case.
No technology is perfect, but imperfect technology that lets you Get Stuff Done(tm) goes a long way.