The answer to your question lies in the roles you want your volunteers to be doing.
Often, when faced with plans or demands for more volunteers, organisation jump straight to the question of recruitment – how can we attract people to come volunteer with us so we can increase the number of volunteers we have. Sometimes this focus is on the message and other times it is on the medium, as you seem to be doing.
This jump straight to recruitment is, in my experience, tempting but flawed. It can lead to recruitment messages that don’t relate to what people will actually do (leading to dissatisfaction and turnover) and embracing new media without a clear idea of why or what result is to be achieved.
Much better is to do some focused work identifying the roles you want volunteers to perform within the organisation and considering who the ideal people would be for those roles. This then helps you to target your recruitment messages much more effectively, often resulting in diversity because of the specific requests made to people to give their time. It also enables you to select the most effective mechanisms for reaching people, be that online, face-to-face, posters etc..
I always remember Fraser Dyer talking about organisations’ who recruit in doctors surgeries and libraries because they are good placed to get volunteers. He wrote that without a clear idea of whether the people you need volunteering with you are likely to be found in such places they are in effect like the hands of a broken clock – they tell the right time twice a day but aren’t the best way to tell the time. In other words they may get you some volunteers but aren’t the most effective approach you could take.
I hope that’s of some help.