This is a case of overthinking a situation. If people want to make ‘shithead’ jokes, having a space between the words isn’t going to stop them. This level of crude humour has no regard for grammatical correctness.
If Facebook was worried this was an issue, the solution would be to find a different name entirely, not debate over compound words. This article puts too much weight on the importance of good brand names. In the vast majority of cases, products survive or die based on their feature set, not on their names.
Case in point: the iPad.