You make "it's not Facebook" a key part of your value proposition. This was the case for Snap early on, and I believe it's a big part of the reason why they got so far. Their users needed a social network separate from their parents, and they had been trained to fear long-term memory upon which Facebook had built their product and their business model. WhatsApp was also successful in warding off FB Messenger's efforts to clone it, in large part because it offered independence and a very un-Facebooklike product experience.

Facebook has shown an impressive flexibility and willingness to pay the costs of making big changes when they need to, e.g., acquiring WhatsApp or shifting their product toward Stories. Still, a startup go a really long way by building a product that carries "not Facebook" as a key value proposition.