Lowcountry beaches are barrier islands, really not any more than big sandbars sifting and sliding through the surf. They don't sit still. Sands erode and pile up downstream, moving the bar.
Beaches near inlets - most Lowcountry beaches - are notoriously prone to accreting, adding sand, or eroding, losing it. Folly is among the hot spots for erosion. It's beaches tend to erode 2-4 feet per year, "although in recent years the northeast end of the beach (near the Morris Island Lighthouse) has been highly erosional," S.C Health and Environmental Control noted in 2009, in its most recent State of the Beaches report.
Meanwhile Sullivan's Island and much of Isle of Palms are accreting sand, except for their highly erosive inlet stretches. The wild card in what's happening here is the Charleston jetties. The jetties are rock walls lining the shipping channel that interrupt the predominant north-south flow of sand in the shore current, piling some of it back on Sullivan's Island and possibly backing up flow to add to Isle of Palms sand. That sand otherwise would be headed toward Folly Beach.
Nobody really knows how much sand that robs from Folly's eroding beach. Estimates have ranged from about one-third to well more than half, but coastal scientists readily concede it's hard to gauge. If the jetties were not there, Folly would be another barrier island in the wash of the longshore current, with its spits being cut and carved by inlet sand shoals as they move ashore.
But the estimated effect is severe enough that Folly Beach won the 50-year settlement in which the federal government pays the bulk of its renourishment costs.