Source : http://www.digitaldoughnut.com/home-content/home-content/email-marketing-social-media-marketing-online-reviews--which-gives-the-best-roi

In this article, it was explained that each marketing tool has its specific function :

"People are still reading email and marketers are still using it (66% say email gets a good ROI), but to stay ahead of the game and get the most out of your investment in email you need to be on the ball:"

"...social media is certainly a cost effective marketing tool. When it’s done well, it’s also a great way to encourage users to convert – 46% of consumers rely on it when making a purchase."

" Recent research by online review solution Trustpilot, shows that reviews have the ability to increase business revenues by 40%."

Yes, every single media has ability to contribute in increasing ROI. But, from my point of view, it has a particular marketing characteristic. 

Start from Email Marketing. 

In this digital era, who doesn't have an email? People create and use an email account to communicate with others. Yet, in business environment, email could be used to manage customer relationship. When we need to introduce our product, develop the brand awareness, and communicate with our client, Email Marketing takes its role professionally.

Then, we are going to talk about Social Media Marketing.

Young people use social media to have a social communication with their friends in cyberspace. For some company, social media enacted as a cost effective marketing tool. Their necessity to promote their product in a free platform can be filled by social media. If your target market is young generation, do the marketing communication through social media. Trust me, it won't lament it. 

And the last, Online Reviews.

As I researched before, online reviews are the most powerful tools to strengthen our consumer's willingness to buy. Through online reviews, our potential buyer would be know our real capacity in creating product or serving the service. Good comments from past transaction would be increasing our credibility. But, on the other hand, a negative comment from one people also could make other reviewer query our product quality. So, how could we manage it well? Do not ever neglect any negative comment. If you get one, just give a response, say thanks for the negative comment, give an explanation, and say sorry for the inconvennience. Ignoring a negative comment would make your professionality going down.

That's my inference about article which I read last night. So, which one do you prefer?