That's not an environmental impact at all. Hardly. What you're asking is: "What's the environmental impact of employing more programmers?"
Sure, let's say worst-case, facebook hires programmers that would have got another job elsewhere, that other company must now hire different programmers and it trickles down eventually until some people who were going to be unemployed get a job programming somewhere. Okay, so now those people have a job. Let's ignore the fact that you're essentially saying that we should cut down emissions by making people unemployed.
It's true that having a job will probably cause them to contribute a bit carbon emissions than if they were unemployed, but studies have shown that the large amount of our personal environmental impact is non-reducable (around 50%), even if you were to go homeless and live on the streets, we might see maybe a 10-20% increase of their personal carbon emissions. For even a couple dozen programmers that's nothing compared to running 20,000 more servers per year and that's absolute worst case.
What you're arguing is the economic cost for facebook, whether it's worth all the man-hours to make their severs more efficient. That's debatable, but if you want an example, you just have to look at Google where they have a team that is devoted to increasing efficiency, despite the fact that they are already running some of the most power-efficient servers in the country.