We just noticed this last night so it's likely caused by a change in how Facebook handles the Open Graph data:
If you enter a Facebook Account ID in the plugin, all Like buttons on the site stop working - that is to say when you click Like, the number goes up one and then down one almost immediately. In other words no Like is registered. By taking the Account ID out the like button works again.
Moreover with the Account ID inserted, the Facebook Developers Lint Tool returns an unknown error:
I'm guessing this is just Facebook being up to no good, but it's an issue that will appear for anyone using the plugin so it should be dealt with swiftly. I've tested it on 5 different sites and they all have the same problem.