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.

Web Design & Development Glossary
AJAX
asynchronous JavaScript and XML. A technique used in web page development.

Web Design & Development Glossary
API
application programming interface. A set of rules programs use to communicate with each other.

Web Design & Development Glossary
color stop
A special element that indicates a color to be added to a gradient.

Web Design & Development Glossary
FTP
File Transfer Protocol. A network protocol useful for transferring files in a client-server relationship.

Web Design & Development Glossary
HTML
HyperText Markup Language. The predominant language for building web pages.

Web Design & Development Glossary
HTTP
HyperText Transfer Protocol. The primary networking language for the Internet.

Web Design & Development Glossary
PHP
PHP Hypertext Processor. A scripting language that works well within HTML.

Web Design & Development Glossary
socket
A technology that allows remote computers to maintain a persistent connection in order to communicate with each other.

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sprite
An graphic object on a web page that will be manipulated in real time.

Web Design & Development Glossary
SQL
Structured Query Language. A programming language useful in managing relational databases.

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stateless protocol
An Internet procedure that completely breaks the connection between the client and the server after a transaction, meaning that the next transaction will require an entirely new connection.

Web Design & Development Glossary
Telnet
A network protocol useful in interactive, text-oriented communications.

Web Design & Development Glossary
W3C
World Wide Web Consortium. The organization that sets international standards for the World Wide Web.