There is confusion about the relation between these two and the misunderstanding that communities are not social media. Not so. Social Media is a form of electronic communication that consists of different platforms; communities are one of those platforms. How do you want to engage?
Social Media defined by Merriam Webster:
Forms of electronic communication (as Web sites for social networking and microblogging) through which users create online communities to share information, ideas, personal messages, and other content.
Online Communities defined by CommonCraft.com:
An online community is a group of people with common interests who use the Internet (web sites, email, instant messaging, etc) to communicate, work together and pursue their interests over time.
Social networks like Twitter, Instagram, or Facebook are fun and sexy: they are used for social listening, brand promotion, and limited customer engagement. Communities are electronic Town Halls that enable conversations and deeper engagement: customers provide feedback, comments, and questions; brands have obligation to respond.
No matter brand promotion, customer service, or customer engagement, you must understand your audience: what networks are they using, and where you are comfortable engaging. As with anything: you need the right tool for the right job. For social media, you need the right network to reach customers and have the right conversations. Communities are social media.
Where do your conversations happen?