Last principle of social media is that it is a very powerful tool to engage with your donors. Not only will you reach new target groups when using social media, but you can also involve them in your work. You can react, reply or send them private messages.
A good channel to reach out for a repeat donation is by using the same medium that they came into contact with you. So if your supporters were reached through email marketing, use email to contact them again, for example to ask them for another donation. Remember: it is easier to retain an existing donor than acquiring a new one!