Validation of ideas in the traditional way would include putting down ideas on the paper, sharing it with friends and families and asking them questions, finding out if your ideas would work, getting opinions. Next step would be to take it to proper domain or industry forums and pitching it. Asking experts within them. Finally asking some potential customers and see what they think.
Now as to Social Media tools, you can do all the above much more easily and then on a larger scale with less time and cost. You can do so without geographic limitations too.
Everyone wants to be heard. So with right set of tools and right target audience, you can get to hear a lot of perspectives and unique knowledge which can be useful to validate your ideas. You can share your idea or parts of your ideas using your social media profile and let people comment on it.
You can also use some incentives to motivate people to participate. You can reciprocate in a befitting way to encourage and build your trust at the same time. This can be used even within an organization.
Online surveys is one example of this. The surveys could be done for anything from product research to identifying trends to profiling customers.