The readership of poetry in historical terms. This is my sense of things:

In the 1920s the Great Moderns had very small audiences. This poetry was not yet taught in Universities. There were a few literary magazines of small circulation. Poetry readings were rare.

This situation continued. A few poets like Eliot and Auden gained more celebrity in the 1940s, but were read mostly in intellectual circles. Dy

With the G.I bill and the New Criticism, modern poetry was taught more in the Universities after WWII. Poetry readings were still rare. The academic poets of the age (Wilbur, Shapiro) had a small, university-based audience. Dylan Thomas makes a splash.

With Allen Ginsberg and the Beats poetry began to reach a youth audience. Poetry readings became more prevalent. John Ashbery, returning to the States in the mid-1960s after a decade in Paris, noted a huge change. Lowell and Plath, with a more confessional mode, helped to popularize poetry as well. There were poetic movements associated with social movements: feminist poetry, ecological poetry, black poetry.

Creative writing program expanded in the 1970s, creating a vast supply of poetry, as well as an increased demand: students studying creative writing. (Supply increased faster than demand, but both increased.) John Ashbery's Self-Portrait sold more than 20,000 copies, a number that would have been unthinkable in the 1920s for a difficult modernist poet.

Things continued more or less along the same path. Popular, elite, and middle-brow forms of poetry co-existing. A small but vital poetry blog culture emerging in 2002. All the audiences that have emerged since the Beats have more or less kept going in some form, with new venues and audiences emerging. I'm sure even a mid-list poetic author of today has more of an audience than Wallace Stevens in the 1930s. Hell, I have more readers than William Carlos Williams in the 1920s.

Confessionalism, beat rebellion, expansion of higher education, and middle-brow media like NPR have "grown" the audience for poetry, which was relatively miniscule before Howl and Life Studies. May Million Poems have a million readers.