Before anything else, you'd need to create a solid social media marketing plan.
Your social media marketing plan should include your goals and objectives. Once both are set you can start setting up your KPIs and metrics.
If you don't have any numbers or data to compute yet then you'd need to start doing so before you set your benchmark.
After setting your benchmarks for the KPIs you've set then start measuring.
You will then know that you are achieving success if you are hitting or is above your benchmarks.