Personally I like the advert, I think it's positive, quirky enough to gain interest, and has most of the information I would need. However I have a couple of thoughts:
To reach people, I would suggest that SO jobs is not your best bet. Purely because it is so heavily weighted to developer jobs that security folks don't use it. Instead, I would suggest professional networks are the way to go. LinkedIn is an obvious one, and I'm more than happy to share the advert with my network as around 1400 of my contacts are senior security folks [status-completed]. You could also sponsor or attend the odd security event or conference - this is after all where security people go, however different conferences have different audiences, so choosing the right one will be essential.
Another option is the professional headhunter route - pretty effective, and I have used this in the UK, but it is a cost option so perhaps wait until networking fails.
Typically, I'd expect the Director of Security in an organisation the size of Stack Exchange to be, in effect, the CISO, as they will have to shoulder the responsibilities and accountability of a CISO, especially as regards the New York State Legislation on named individuals taking responsibility for Cyber Security (currently only mandated for banking, insurance or FS, but likely to spread further). So this role really needs to be a peer of CTO and CRO, or at least able to have a seat at the top table on security decisions, otherwise they will not be empowered at a sufficient level and will still be blamed for failures in security.
The advert doesn't make it 100% clear that this role could be anywhere. It sounds very US based. Those of us who know Stack Exchange well have seen how well you work across the globe, both remotely and office-based, but I think you could perhaps expand on the non-US possibilities for those who may not be as familiar.
- Existing In-House Security Experience
Another small thing would be to give an indication of the existing security expertise. While you may not have dedicated security staff, it can be very reassuring to candidates to know what they have to work with. Sure, this can be discussed in interview, but it would be good to know what experience Jarrod, Mark, Geoff etc have in security.
Happy to chat further on any of those on my usual email if you need.
p.s. To solve the Rory issue, just make a renaming ceremony part of the onboarding process :-)