I'm trying to encourage people to share posts from my blog on social networks. To that end, i recently (past couple days) added a share prompt to each post. When the user has scrolled to the end of the post, a "share this page" popout slides in from the left, at the bottom of the screen. It automatically disappears when the user scrolls some more.
Is this the best time to show the prompt?
Would it make more sense to prompt the user before they've finished reading the entire post? (Say, after 3/4.) After all, not everyone (read: most people) reads the whole thing, and some are willing to share anyways.
However, i feel like sticking a big "please share this page" in someone's face is disruptive to those people who actually are planning to read the whole thing.
To maximize both UX and marketing, what is the best time to show the prompt?
Here's a mockup of the layout. The green rectangle is what is shown on the screen, i.e. where the user has scrolled to: