Here are two recent papers that measured the "impact of social media on brand loyalty". Some scales are used that you might adapt or get idea to design yours.
Erdoğmuş, İ. E., & Cicek, M. (2012). The impact of social media marketing on brand loyalty. Procedia-Social and Behavioral Sciences, 58, 1353-1360.
Laroche, M., Habibi, M. R., & Richard, M. O. (2013). To be or not to be in social media: How brand loyalty is affected by social media?. International Journal of Information Management, 33(1), 76-82.