Dan,
We do still support Joomla 1.5. However, we are no longer adding new features to the Joomla 1.5 version. The last release for Joomla 1.5 was JFBConnect v4.3.4. We will continue making bug-fixes for that release, and providing support for issues, feedback, or implementation help for the remainder of this year.
The newest release, v5.0.0, is only available for Joomla 2.5 and 3.0.
Unfortunately, the Joomla Extension Directory removed the "Joomla 1.5" tag from all listings, so you won't be able to tell from the JED which extensions still support Joomla 1.5.
I hope that helps explain, but if you have any other questions, just let me know.
Thanks,
Alex