There are a number of different ways to measure the effectiveness of social media, but one of the most common is through the use of KPIs (Key Performance Indicators).
KPIs can be used to measure a number of different metrics, but some of the most common include things like engagement (likes, comments, shares, etc.), reach (the number of people who see your content), and leads (the number of people who are converted into customers or clients).
There are a number of different ways to set up KPIs, but the most important thing is to make sure that they are relevant to your specific goals and objectives. If you're not sure where to start, there are a number of resources (like this one) that can help you get started.