Over the past few years, it's become common to use your Google, Twitter, or even Facebook account to sign up for other services. This speeds up the sign up process as you don't need to fill in any sign up form, nor do you have to open up your email client to click on the verification link before your can actually use the service. It also removes the necessity to choose and remember yet another password, although this is not as cumbersome anymore if you use a password manager.
However, this convenience comes at the cost of greatly multiplying the consequences of a hack. Indeed, say that you use your Twitter account to log into 10 additional services. If your Twitter account gets hacked, the hacker will not just have access to your Twitter account but also to those 10 services.
Therefore, it is even more crucial to secure these “foundational” accounts, i.e. the accounts that you use to log into other services, with multi-factor authentication. 2FA with SMS is insecure so you should use something like Google Authenticator that implements TOTP or HOTP.