How long should a Facebook post, a Twitter post, a headline, or an e-mail subject be if you're looking for maximum engagement?

[youtube http://www.youtube.com/watch?v=0e466vrA59w&w=600&h=315]

You can read the full post with methodology and outbound links here, or check out this infographic for a condensed version:

This is mostly designed for marketing through social media / web / e-mail, but I'd probably disagree on the 18-minute presentation. I think that's too long. If you're g…