Social media companies are especially doing this whiplash switch from aiming for growing the userbase to making money. And for them, there is another factor that makes it even more important to use money for growth when it is available -- network effect. Basically, for certain services, the role of the service is to facilitate communication between their users. While it's not quite true that all users are equally-likely to communicate with each other -- an elderly user who only speaks Italian and a schoolboy in Kansas who only speaks English might not have a lot of desire to communicate -- in general, users of the service get their value from the service by communicating with each other and each additional user is one more person with whom a user can communicate. This means that it's much more-desirable to use a service with a large userbase than one with a small one, because you can communicate with others. The value of the service as a whole, if everyone were equally likely to communicate with everyone else, rises roughly as the square of the number of users. That's because the value to each user is proportional to the number of users that they can talk to, and that is true for every user -- multiply one by the other, and the value of the service as a whole is proportional to the square of the userbase size.
Social media work by connecting members of their userbase. So for them, they have a huge incentive to use money for growth whenever they can get a hold of it as far as they can.
The services that are especially likely to respond to capital being cheaply available are companies that have a business model that does this, even moreso than a typical dot-com. And sure enough -- Twitter, Reddit, and YouTube derive their value from connecting members of their userbase, rely on network effect as well as economies of scale. And just as they dive really deeply into spending cheap money to grow when they could, when money ceases to be really cheaply available, so they will have further to swim out when it ceases to be.