A big shout out to ADOBE for helping with this issue! Not! Unless I doing something wrong, this is extortion! They should not tell you that there is a free upgrade and then after the upgrade the plugins that you had do not work anymore....oh but wait, they will if you PURCHASE the plug in???
Adobe has a FB plug-in that comes with LR. This plug-in may or may not work depending on what changes FB makes. LR 5.6 was released the end of July which is more than 2 months ago. FB isn’t going to stop making changes just because Adobe releases a FB plug-in. They are both independent companies with their own market pressures and productions schedules.
If you go to the plug-in exchange as it seems like you might have, there is a FB plug-in or two sold by other vendors, perhaps free perhaps not, but they are not the Adobe plug-in, and they may or may not work depending on what FB does.
If you want to roll back your LR version to a previous LR 5 version, the install files may still be in a desktop \ Adobe folder, but if not just go to the Adobe Updates page, find the link for LR updates on your computer platform, Mac or PC, and download and install the older version. If that older version installer says you already have a newer version installed, just uninstall LR 5.6 and then install the older LR 5 version.
My view of the situation is that if you are relying on a FB plug-in to LR then you probably should actually buy one from another vendor that has a more frequent update schedule than LR which is 3+ months between versions.
I’d don’t know anything about the vendors listed on Adobe Plugin Exchange, but I have used plug-ins that upload to others services besides FB, from Jeffrey Friedl, that seem to work great and he does update things as soon as possible when something doesn’t work: