You're asking why so many people seem to be addicted to social media?
The initial reason people join sites like Facebook is to keep in touch with their friends, classmates, or colleagues. Many people use social media sites for this, as opposed to sending many individual e-mails.
And then, the way these sites work, your group of "friends" will continue to grow, as your "friends" link to their "friends."
Also, many companies, universities, or any number of other organizations, now have a presence on social media sites, so you can also stay up to date with news from these sources. These social media sites become a way of staying up to date with your contacts and your interests, without having to search multiple sites and send multiple e-mails.
I agree with your convincing point that ' social media sites become a way of staying up to date with your contacts and your interests, without having to search multiple sites and send multiple e-mails.