The K2 Facebook buttons should not be used in conjunction with JFBConnect. K2 still includes the older, deprecated all.js Facebook javascript library and JFBConnect uses the newer recommended sdk.js library. When both of the libraries are used at the same time, the Facebook buttons will not load or work properly on the page. This is why you're seeing the buttons not show up in K2 when JFBConnect is enabled.
If you would like to add the social sharing buttons to your K2 content, please enable our JFBCContent plugin and configure the appropriate K2 Like and Comment tabs in the JFBConnect > Social configuration area. Additionally, disable the social options in your K2 items.
I hope this helps clarify the problem, so let us know if you run into any further issues.
Thanks,
-Melissa