Lead acid batteries last longest when they are fully charged and kept that way, and discharged infrequently. This makes them excellent for use in standby power situations, where they are almost always topped up ready for the power to go out.
Li-Ion batteries last longest when they are actively used. Keeping a Li-Ion battery fully charged all the time is bad for its longevity; the battery structure breaks down faster at a high state of charge. This is why it is recommended to store Li-Ion batteries half-charged in a cold environment, and why cars like the Tesla Model S normally only charge up to 80% unless you require a "full-range charge" for a road trip. Not topping off to 100% extends battery life.
Maybe Facebook intends to keep the batteries at 80%, but it's hard to believe the economics are going to work in their favor.
Not to mention that lead-acid batteries are mostly water and non-combustible sulfuric acid. A Li-Ion battery fire is 50 times nastier than a lead-acid battery fire, and produces a hell of a lot more noxious gases.