With the election only a week away, the political parties' use of targeted advertising is well and truly a headline issue. This is a very challenging area in terms of the data protection driven requirements to achieve transparency (as flagged by the ICO's investigation launched last month), and the practical aspects of raising awareness among the general public. Data analytics practices can be incredibly opaque – it would be almost impossible for most people (even relatively tech-savvy sections of the public) to understand let alone challenge the algorithmic decision making processes involved.
Even though this is arguably becoming 'the norm' in political campaigning globally (the 2008 US Presidential election is widely referred to as being the "Facebook election"), the act of 'profiling' and then targeting an individual through analytics (which is essentially what is happening here) is considered to be relatively 'privacy intrusive', and so the political parties are ultimately going to be pushed into finding a way to make sure that they're tackling the legal issues and protecting the rights of those individuals that they're profiling and targeting.
In practice that means that the political parties must find a way to meaningfully notify individuals about the sources of the data that they collect data and what kinds of profiling and targeting they carry out. The parties will also need to think about their legal 'hook' or justification for carrying out this relatively intrusive activity – whether that's consent (which, in practical terms, is likely to be quite difficult to obtain) or another legally sufficient justification.
Facebook clearly also has a role although, technically speaking, the primary legal responsibility here lies with the political parties – they are the organisations initiating the analysis and targeting, albeit using Facebook's data, algorithms, and ad targeting/delivery capabilities (interestingly, Facebook markets its political advertising services: https://politics.fb.com/, a factor that potentially blurs the lines and means that there are good arguments for requiring increased transparency from Facebook, too).
The reality is that while the political parties are the ones primarily 'on the hook', they will be almost entirely reliant upon Facebook and the analytics providers being willing and able to explain some of the profiling technology/algorithms so that the parties can in turn meaningfully inform (and protect the rights of) the individuals being profiled and targeted.
Although this is arguably becoming the norm in political campaigning, profiling and then targeting an individual through analytics is considered relatively privacy intrusive, and so the parties are ultimately going to be pushed into tackling the legal issues and protecting the rights of those individuals that they’re profiling and targeting.