Forgive me jumping in here, but surely, if your host is pointing the finger at EasyBlog, the sensible thing is to not only give Mark the logs, but also give him FTP and back end access so that this can be investigated. IF the hackers did use EasyBlog as a means of entry, then wouldn't the only logical thing to do is let the stack guys investigate? After all, who knows their code better than them?
If you've been hacked once, you'll probably get hacked again, unless preventative action is taken and IF EasyBlog has a vulnerability, let the guys look at it and fix it. There are 1001 ways your site could have been hacked and I wouldn't be so quick to point the finger at any one area or extension without a thorough investigation.