If you're planning to advertise (or promote) your Facebook posts, there are additional restrictions you need to follow to keep Facebook from rejecting your ads. The biggest of these is the 20% text restriction.
Although this limit is listed in Facebook's requirements, they don't explain how they calculate it. To calculate the text percentage, Facebook draws a 5x5 grid over the portion of the ad visible in the timeline view. If text covers any portion of a square, the whole square is counted. If your text covers more than 5 squares, your ad will likely be rejected.
To make sure my ads are compliant, I've created a PSD with guides dividing the center portion of the image into a 5x5 grid which you can download here. If you make sure your text only covers 5 blocks, you shouldn't have any problems with ad rejection.
Click here to download the Ultimate Twitter and Facebook Image Template for Advertisers.
Note: I recommend setting the template file to “Read-Only” to keep you from accidentally saving over it. Once you’ve made your changes, just make sure to click “Save As.”