Materials used for small drones have a very difficult time scaling up to very large lightweight structures. Care to expound on the loads and stresses experienced by a 144 foot wingspan wing, that weighs only 900 lbs? This drone was built to be light and barely strong enough. That was its design point. Have you shown the world that you can do better and can credibly criticize their design?
Extremely high aspect ratio wings, like this one, just don't want to quit flying. It is a real challenge to bring it down onto the ground. The pilots possibly exceeded design Vne trying to get it down. This exponentially increases drag forces on the airframe. Ground-effect makes landing even more difficult, and with a 144 foot wingspan, ground-effect starts at ~72 feet above ground.
Add to this the non-homogeneous nature of gusting winds in proximity to the ground, and it is not inconceivable that design limits were exceeded by a fluke of nature. There is not enough public information of what weather conditions existed during landing, but sudden adverse conditions during landing are not unprecedented. Did Lockheed's engineers under-design the structure on the L-1011?
Disclaimer - I work for an aerospace prototyping firm (not related to Facebook), and have worked on things that cover the conceivable gamut of Reynolds numbers that can be "flown". Although I have not worked on Facebook's drone, I have an intimate knowledge of the modern materials used in this air vehicle. My opinions expressed here are my own.