K2 v2.5.x is definitely supported, as we're using it on our blog on this site as well. We're not on Joomla 1.7 though, but that shouldn't be the problem.
Can you let me know the following:
* Make sure the JFBCContent - Content plugin is enabled.
* Does the comment box show up in K2 if you have the position set to Top, Bottom, or Both (try them all)?
* Do any of the social buttons (Like, Google+1, Twitter, etc) show up in K2 if you have the position set to Top, Bottom, or Both?
* In a normal Joomla Article, if you set the positions for either the social buttons or comments box, do they show up in the article?
If it doesn't work in any of those cases, can you post (or Private Message) the URL of a K2 page on your site that it won't display on?
Thanks,
Alex