There is a major difference between agreeing not to hire from each other, and just not actively going after each other's employees. I have worked at Google in the past, and while I was working there I was actively being approach by facebook recruiters. I was not the only person being approach like that. I heard from multiple colleagues that they too were being contacted by facebook recruiters. I am aware of multiple of my former colleagues who actually did get jobs at facebook.
To me facebook felt too aggressive in their pursuit of Google engineers. There was a real practice among facebook recruiters to search through linkedin and any other source, where they could identify Google engineers and contact them. I saw enough evidence pointing towards those people being contacted simply because they were currently working for Google. Personally I had zero interest in switching from Google to facebook, I don't think I even bother answering, when I was approached by facebook.
If Google and facebook had reached an agreement under which facebook would be a little less aggressive in their pursuit of Google engineers, I would not have feared this would have a negative impact on my salary. And it would have felt a bit more reasonable to me. The recruiters could still look for talented employees, and if by chance they end up finding Google employees, they could still approach them just like they would have, if they had not been Google employees. As long as they weren't directly picking candidates based on them working for Google, I would call it an improvement.
Some people have argued those companies shouldn't even actively be contacting candidates. Instead they should wait for interested candidates to submit a resume on their own. Even that would not even get close to not hiring each other's employees.
Employees could still move from Google to facebook, they just had to take initiative to submit a resume. I would only consider there to be a real problem, if facebook would reject resumes submitted by candidates, just because they happened to work for Google. I have seen no evidence of such a practice existing.