You seem to have fallen into a classic pitfall of reporting on others' misreporting of statistics, in that there seems to be a quantity / quality confusion here.
These numbers need context, which neither the BBC in their article, nor you in your criticism of it are providing. You have relied on a quantity based analysis of rape murders, which I find implausible.I'm not sure how many teenage murder rapes* this button would need to prevent before it was worth the trivial sounding hassle to create it, but I hope we would all agree on something less than 200, even if that is 0.0001% of total UK Facebook users**.
The BBC has vaguely hinted at a quality based analysis "There have been some very serious allegations", which lacks supporting evidence. It could perhaps be criticized on those grounds, but not, I'm afraid, on the grounds you chose.
* which I presume is only the most extreme kind of crime being tackled (effectively or otherwise) here
** a potentially misleading raw number; I am one of those 28 million, but I have no need for this button as there is no possible way for anyone to do anything bad to me via social networking.