Using Social Media Metrics to Choose Placement for Your Call-to-Action
It's important to use metrics to help with the placement of calls-to-action. Do A/B testing on page placement. Run versions of your landing page that have the button(s) in different places. Try running two versions at a time; some people run as many as five test placements at once, delivering randomly.
Your metrics can help you determine which versions get the most clicks overall, which parts of the page get the most clicks, and which button colors and placements get the most conversions from clicks (for example, filling out a form or other further action).
If you're using some of the new page insights, you can even look at click percentages on the pages you're testing. This number not only gives you a visual of the data for your button placement, it puts it in context with the other clickable elements on the page: navigation bar, logos, creative content, and other calls-to-action. Having that perspective will help you make a more educated decision on which version of your page to use.
Some people find that running different versions of the landing page with calls-to-action for each incoming link source is effective. Just as you design a landing page for, say, a Twitter audience, you can tailor the buttons on that page as well. There is nothing wrong with keeping that A/B rotation up permanently, triggered by referral source.