Huge productivity enhancer. You can always collaborate with someone either over the net or by just walking over and talking in the same office.

While in open plan you're always "collaborating" with everyone within earshot (I also get distracted by people moving around behind my screen).

There's a reason why open plan was mostly abandoned in the 60s, an no good (non short-sighted) reason why it was resurrected in the '00s.

What was the actual reasoning? Like, Zuck is a survivor and genius I respect (he's not perfect tho obviously)...but why does he do open plan at FB? Why did any of the tech companies do open plan? Are they thinking about it? Do they measure it? What's the rationale?

I'll assume you mean offices rather than WFH, since the latter (and its many confounding factors) has been discussed extensively here already. Personally, I was just happier in an office, even if shared with one other. Cubes were second, open plans last, though there were a few times when I appreciated being "in the fray" with everyone else either way. Eliminating the constant sensory assault just makes focus easier, and at the end of the day I could go home with lower cortisol levels. Collaboration was never an issue, as long as there was some space nearby for it and people were willing to use it.

Obviously it can be easier to slack off in an office, or there could be conflict with one's office-mate, and so on, but overall I think offices were worth it. I've also been told by multiple CFOs that the reason for cubes and then for open plans has mostly to do with cost and how various kinds of space are taxed. Every one admitted that the productivity/collaboration claims were just excuses. Which brings us back to WFH vs. RTO, I guess. ;)