Multi-tenanting occurs when there are low costs or no costs to simultaneously participating in competing networks at the same time.
This is fairly rampant with social networks. People often syndicate photos or snaps that they share to multiple platforms at the same time (Facebook, Instagram, and Snapchat). It's also frequent with marketplaces; many people on both sides of the marketplace often use both Uber and Lyft, both eBay and Craigslist, both Etsy and Amazon Marketplace.
Multi-tenanting can harm the defensibility of a network and mitigate network effects, but only up to a point. Ultimately, the bigger network will win out because it will have a higher profile for potential new customers and will be more likely to retain existing customers, even if those customers occasionally multi-tenant. Because the network is bigger, the number of options will be greater and people will only turn to competing networks in moments of dissatisfaction or as a supplement for the value provided by the larger network.
For example, people who use Snap stories may use Instagram stories to reach a different audience. Usage of one service may serve to reinforce and increase usage of the other service as people become more habituated to using both and see greater value from the broad category of "stories" shared on both.
Customers of eBay or Amazon might look to Etsy specifically for vintage products, and although this is a case of multi-tenanting, usage of one reinforces the other because the greater the variety of goods consumers buy online, the more likely they are to shop online.