Hi there,
This page from Facebook will walk you through the process. Once you have the Share button looking the way you want it to in the configurator, click Get Code and then add the provided code blocks to the page.
There's a Javascript or an iFrame method of adding the button to the page. The iFrame method is faster. Simply grab the code and paste it into a Custom HTML element in the Weebly Editor, then drag that element to the place you'd like it to appear on your page.
If you'd like to use Javascript, you can add one part of the code to the <head> section of your page either in Settings > SEO or in the individual SEO Page Settings. After you've done that, copy the secondary piece of code into a Custom HTML element in the Weebly Editor and drag it to wherever you'd like it to appear on your page.
Hope this helps!
- Joe
