I don't think you can re-assign the properties of the storeFeedJSON struct. storeFeedJSON is the result of your cfhttp, right? as in
<cfhttp url="xyz" result="storeFeedJSON">
... which I think is a protected or immutable data structure. You can obviously pass it as an argument for your function, but put the output in some other variable, or perhaps straight to the json deserialize.
<cfset local.cleanFileContent = process.CleanBadUnicode(storeFeedJSON.fileContent)>
<cfdump var="#deserializeJSON(local.cleanFileContent)#">
... and while I'm at it, you might want to use getasbinary="never"
because Facebook may someday give you binary data, or change its headers or something, and then your code will break. If you have to receive binary, you'll need cfhttp.fileContent.toString()
or something like that.