I'd suggest to disable the Facebook account in your Aperture preferences - remove Facebook from the "Web Accounts" panel in the Preperences panel.
See this paragraphs in the Aperture User Manual:
Then export the images your want to publish to Facebook and use Facebook's uploader. When exporting versions from Aperture, you can decide, if you want to add IPTC and Faces data.
Hi Léonie! Thanks for this. And it looks like specifying export options appears in 3 places. (The Export dialog, the Export preferences, and the Export presets). Is IPTC specified through "Include Metadata" in the image export presets? (I found Faces in the Export preferences).
It looks like the only way you can get a Facebook caption using Export, is through the Version Name, and exporting with Version names, as Facebook uses the file name as its caption. Is that right? (I think that's what you would do with Facebook sync too).