It's true that Facebook didn't commit to maintaining these projects on an ongoing basis, but that has nothing to do with the fact that they did release lots of major projects under an open source license, projects written by highly skilled engineers that could have been kept internal.
You're free to fork these repos and maintain them yourself if you'd like, and that freedom exists only because the code was made available under a FOSS license in the first place.
The alternative to this situation is not "open source code maintained in perpetuity", but "code that was never made open source in the first place".
There's no obligation for maintaining or ensuring that the open source code remains useful to you, so the criticism that projects are being open sourced and ignored might be valid, but the criticism that Facebook doesn't release major projects as FOSS is not.
We've all been spoiled by the hard work people have been putting in maintaining their FOSS repos, to the point where we now expect it. Most FOSS licenses include wording such as: THE SOFTWARE IS PROVIDED “AS IS” and that's what these repos are: code provided to you as is, free of charge and no strings attached (other than what the license specifies).