Why is social media not used by governments so we can cast our votes on big decisions?

[David]

A large portion of country populations are on social media now, so why don't governments use this to get us more involved in big decisions? So rather than guessing what the people need or want they can simply cast a vote.
Obviously the government would have to set up a site for this to be done and entering your national insurance would ensure you only get one vote.
For those who don't have internet access there are library's and if an idea like this was taken on board then I'm sure councils would allocate areas where people can have there vote.