Security is a very important aspect with any website, though with a social networking website it is extra important, as we will store personal information of lots of users, so we need to ensure we keep this data secure.
Data Protection legislation
It may be worth investigating the Data Protection legislation in the country you reside in, and in the country your website is hosted in. As well as mandating how data must be kept secure, some countries, such as the UK, also have a register of Data Controllers who are responsible for data protection in a particular organization.
The security of the server itself is one aspect that needs consideration. This can be broken down into two primary areas: