why? what is the difference between this and Facebook social module?

Hi...

What is the reason for this module?

As per the description, "Rather than having to manually copy/paste the Like this on Facebook code for each piece of content you (or your users) create, this module will automatically add that code to the end of each piece of content for you."

Is there any difference between this and Facebook social module?

Yes. Facebook social module:

• Has no broken uninstall support.
• Does not implement user permissions, even though the changelog says otherwise. See #858156: Support for enabling the Like button through user permissions..
• #1071762: fb_social_like.module function fb_social_like_link() produce incorrect URL (although last I checked, it appeared as though this issue was fixed so I'm not sure what the deal is.)
• Appears to have lots of general issues with the "Like" feature for some reason, which is one of the most prominently used features on Facebook.
• Is supposedly incompatible with the Drupal for Facebook module (#1010624: Not workind with Drupal For Facebook).
• Requires that the module itself be installed (as a prerequisite) before its fb social like sub-module can be installed/used.

That's all I can think of right now... And please don't get me wrong -- the Facebook social module is great, with lots of potential. But ultimately I think the main difference is that the Facebook Like Button module is much more lightweight while still accomplishing the same goal(s). I could be wrong (wouldn't be the first time) but that's just my take on the matter.