In your opinion, How can I measure the success of my social-media marketing efforts?

You need to set your KPIs first, decide on what you want to achieve and then measure, measure, measure. 

Be careful not to use first level metrics like likes or followers as a benchmark, go deeper and examine e-commerce sales, value of a fan, engagement rate etc and ultimately affect on sales.

Through customers engagement & feedback, sales/financial reports, search traffic and exposure.