Walmart has shifted a decent chunk of their business to the internet and that business does suffer from quite a bit of attrition when competitors manage to undercut their prices - and Walmart established itself by stealing consumers in a short time and forcing local businesses to shutter - then consolidating their locations. It is an extremely impressive business and they've been quite careful managing costs to maintain their position, but they could easily lose their dominant position quite quickly. Customer loyalty to Walmart is entirely objective - they offer the best deals. As a company their image is pretty terrible and very few people would weep if they were overtaken.

Compare that to services that FB runs that have years of your personal history carefully cataloged in it - that data can be exported, but there aren't many services where it can be richly imported.

Both companies may seem to be are in fragile positions because of their size and public opinion but I wouldn't bet either of them will shutter their doors anytime soon.