Ahhh.. that makes perfect sense. What's happening is:
1) You have an article. At the beginning, there's a macro
AddFullArticleHere (I don't know what RokCandy macros look like, but you get the idea).
2) The Open Graph - Content plugin fetches the first 50 words from your article by default to use as the description
3) JFBConnect sets the og:description and twitter:description to be "
AddFullArticleHere"
4) Later, the RokCandy Plugin scans the full output of the page and replaces all 3 of those macros with the full article (or whatever the macro is meant to do).
In this case, JFBConnect is doing what it's supposed to and RokCandy is as well. They're just doing it out of order since RokCandy should replace its stuff first.
Your fix of using the {SCOpenGraph description=...} tag is excellent as JFBConnect will always use the tags over what we automatically generate. If you're going to the effort to create a custom tag, it's an obvious assumption that it's better.
The other options to try is to order the RokCandy System plugin to be *before* the JFBCSystem plugin. That may make the replacements it does happen before our Open Graph plugin tries to scan the article. However, there's many places that each plugin can do it's replacements, so that may not help.
Beyond that, let me know what the macro your using looks like. We may be able to help you update the Open Graph plugin to 'skip' that macro. In that case though, the description tag still won't use what happens after the macro is replaced, since the article contents won't be on the page by the time we try to read it.
I hope that helps,
Alex