Social media creates a snowball effect. When one person interacts with your post, it tells the algorithms that your content is valuable, immediately exposing it to more people. The snowball perpetuates.
First, find potential influencers and advocates in your community. They won't always come to you, so look out for them instead. Become aware of how people are reacting to your brand and connect with the passionate ones.
Although you could use various automated tools, if you'd like to do all of this for free, just start a spreadsheet and keep track of your influencers and their contact information (Twitter, Instagram, LinkedIn ...etc). Then, simply reach out via a direct message!
Social media campaign tends to last longer and have a larger potential than a simple social outreach. When recruiting fans, customers and strangers though, you have to think about a mutually beneficial relationship.
Your brand ambassadors should get something valuable, in exchange for promoting your company. Don't be afraid to ask what they might want.
The value you give them does not have to be monetary, but could instead be excuse access to features or events, or even just a public recognition of their value-add to your company.
Often times the best brand champions are not marketing experts, so the company has to provide them with support and clear directions on how to be the most helpful, and the impact their work would create. Make it as easy as possible for them to help you.
Create a community for the brand ambassadors, help them get your news first, and even start a newsletter with the most helpful links they could share and like. You can even use a leaderboard to promote healthy competition among your influencers!