Good questions and I will try to follow up with what Jack has offered.
We have an open registration policy on the site that has some controls of stopping bots from getting in. I recently added a new step that anyone who registers on the site must include their name and contact information. These steps have reduced bots and spammers significantly. I also block many ip blocks outside the USA.
For this issue you are asking about what I see having happened is new user signed up and started private messaging people a spam message. Looks like they sent the same message out to several/many people on the site. I don't know actually how many, but I will try and see.
Does this mean privacy on the site has been compromised? Not from what I can gather. Not anymore than anyone else getting on the site and sending a PM to you.
This person has been banned and their current username and email can't be used on the site. I have turned off new registration for now.
The site cannot stop all spammers. We do a pretty good job, but it is pretty tough. As for hackers, there are all smarter people than me out there hacking into sites. Just ask Target, Apple, Sony, Microsoft and anyone else who has been compromised.
If you are concerned, I encourage you to change your contact info in your profiles. 123 Elm Street works for me! The site generally doesn't hold anything that can't be found in the whitepages.