How can brands determine the return on investment from their social media campaigns"
Salesforce Marketing Cloud prepared the infographic below, with details on how to accomplish the following five steps:

Choose your goals.
Determine which metrics will help you measure your success.
Track your metrics.
Tie your metrics to a monetary value.
Update your reporting?make projections and improvements.

Salesforce Marketing Cloud said in a blog post accompanying its infographic:
Your social media campaign does have a quantifiable ROI from the total revenue directly generated by your campaign. Before using this formula, you have to determine the value of your social media goals. Whether you look at total spend per customer, the number of attendees in your webinar or how many entries you had in a contest, you have to have a goal for each campaign. Once you decide which goals are most important, determine which metrics measure the success of that goal. Social media offers a wide variety of quantifiable data tracked by the platforms themselves, Google Analytics and third-party software that is tailored specifically to your company?s social media marketing efforts. From your data sources–including your marketing automation software–run reports, make projections and find ways to improve. Continue reading the infographic below for more details on calculating your social media ROI.

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