My question then is, very simply, how secure are websites like Facebook, T w itt er etc?
Most things aim at being as secure as is reasonable against a particular threat level, and then fail because they have bugs.
A social networking site should try quite hard not to let anyone steal and misuse your credit card details, and they're probably no worse at this than any other retail sites.
But as far as personal information is concerned ... the entire point of these sites is for you to voluntarily publish personal information! The main issues are
(1) if I publish something on one of these sites and mark it as "visible to my friends only" then how hard is it for the general public to hack their way into this information?
(2) if I decided to leave one of these sites is my information really deleted?
Well, it's easy to answer (2) - even if the social networking site does delete all copies of your information when you leave there will still be copies archived all over the web, so it doesn't really matter what the social networking site does: once you've chosen to publish information about yourself it is visible for ever.
And in practice as some failures in (1) have occured from time to time the sensible advice would be to put nothing on such a site that you didn't want your partner/child/employer/tax inspector, and any possible future versions of these for the rest of your life, to read. But that's just the same advice as putting information about yourself anywhere electronically.