Facebook. The privacy controls are leaps and bounds better than last year. Also they are generally respected better (i.e. if you lock it down it is respected).
The problems, currently, are mostly to do with a) when privacy options are not being respected properly (by accident or design) and b) Facebook not making privacy options clear to "noob" users.
Irrespective; from the view of a security researcher with an interest in social media (like myself) it is a lot harder today to see someones private details than it was last year.
You sound like you're talking about something specific. Could you elaborate, please?