One of the key tenets of the Curve is that you use free to find an audience and earn the right to talk to them again. A content marketing strategy that just finds an audience with no follow-through is not a strategy.
My preference is email, because it is in my control, but it is important to be where your audience is: YouTube, Twitter, Pinterest, Tumblr and so on.
I have long joked that Facebook is different. On FB, you are “earning the right to pay Facebook to speak to your fans”, and their ongoing changes like hiding the most recent posts and encouraging users only to see the posts that Facebook wants you to see are part of this monetisation strategy.
It looks as if Twitter is going the same way. They have recently announced a change that allows users to follow a brand but mute all posts from it, so nothing from that brand shows up in the user’s timeline. It is touted (correctly in my view) as a way of improving Twitter for its users, by allowing them to have an improved signal-to-noise ratio.
Of course, it also means that Twitter will be able to charge brands for bypassing that functionality with a sponsored Tweet.
Earned media turns into paid media (again), and so the cycle turns.