Email is dying because it's free. If you can send an email for free to 100 of your closest friends, instantly, you probably won't abuse the privilege. But someone else will because they might define 'friend' differently than you or I.
100 times 100 is ten thousand. Spam.
So now, people don't reply when you send them a resume, because it costs too much to do that ten thousand times.
Twitter is next. The paradox is obvious: to grow, you need to remove friction from the medium. If it's not easy and free to use, people won't. But then it gets big and it becomes profitable, so people use it too much.
The churn rate at twitter is reported as more than 50%. That's because of lack of friction as well. Easy to get in, easy to get out.
Stamps are underrated. Friction rewards intent and creates scarcity.