When you register a trademark, you have to register the spelling, the font and (I believe) the color. For example, when Mattel registered Barbie, it was the word Barbie in a particular script, and in pink.
Otherwise, every female and poodle named Barbie would be in violation. And that doesn't make sense.

So I think it's safe to mention Facebook. Just don't use their registered trademark to make it seem that Facebook is involved in what you are doing.