How would you make a decision between pursuing a career in community management vs. marketing vs. social media?

The good news is, I don't think you really have to decide between the three, at least in college, as they really are all different types of jobs you can do under the marketing umbrella. Many people just think "advertising" when they think of marketing, but it's really relates to anything related developing and commercializing a product or service in the market. Anything that falls under the "Four Ps" - Product, Place, Price, and Promotion- is considered marketing, and there are lots of different niches in marketing you can go into with a marketing degree including social media management or being a social media community manager. By specializing in digital marketing and communications in college you have the opportunity to do all kinds of copywriting, social media, digital advertising, etc.

To get into your specific question though, a community media manager is someone who moderates discussions between fans, followers or other web users interacting with that organization or blog's community. If you like finding creative ways to initiate conversations, crowd sourced content (like pictures, posts, etc), contests, etc you'd be a great community manager. If you're more interested in the viral aspect of the internet and how to get more followers and "likes", grow a brand's total web presence across different sites, improve search engine performance, etc, being a social media expert might be more up your alley. For either role you can likely work for many different companies managing their social media accounts (either as a freelancer or working at an agency), or you can work in house for a brand getting very involved in all of their digital media across social media platforms and web publications.

I think an exceptional way of showcasing yourself as a competitive candidate in this field is to have knowledge/experience in all three areas :) When I was in college, there was a huge focus on specialization, but now working, I realize there's a lot more value put on individuals with diverse experiences who are well-rounded in more areas than one.

And more than often, if you take up a general marketing role, you'll need to be able to perform tasks in all areas (social, email, community..etc). Good luck!