The moat
Network effects in social graphs, scale in ads targeting, and brand as the default way humans broadcast. Switching costs are moderate (social graphs are portable in theory, sticky in practice).
Pain, risk, or retraining required to leave.
The product gets better as more people use it.
Patents, data, licenses, process knowledge, regulation.
Fixed costs spread over a volume rivals cannot match.
Structural ability to be the low-cost producer.
Pricing power from trust, habit, or identity.