When a company has a total monopoly over a sector, you are obliged to use the service they provide or you will simply go without that service entirely.
Twitter is not a clear example of this, because it doesn't really have a solid monopoly. But Google and Facebook certainly are - there really isn't a competitor to YouTube or Google Search, and there isn't a competing social network to Facebook.
When a company has a total monopoly over a sector, you are obliged to use the service they provide or you will simply go without that service entirely.
Twitter is not a clear example of this, because it doesn't really have a solid monopoly. But Google and Facebook certainly are - there really isn't a competitor to YouTube or Google Search, and there isn't a competing social network to Facebook.