Would you agree that they pretty much have the same focus and outcome? If not, when should one be used over the other?
Influencer marketing and word of mouth marketing are in the same marketing family but are different. Word of mouth marketing is when a friend, family member or somebody you look up to offers a suggestion for a great service or product with little or no incentive. In theory, it is more "organic" as it is "usually" a more genuine recommendation, as the person didn't get anything for the posting the recommendation.
Now Influencer marketing, it is not so black or white. The Influencer may love your product or service, but they have more incentive to promote your product or service, it is called money and/or free products/services. Not to say that it is not genuine but there are incentives for a more favorable opinion.
My analogy would be this:Word of mouth = Organic search on Google (In theory)Influencer marketing = Paid Placement (Pay Per Click)
Now please understand that an influencer can recommend a product or service, get paid and love the product/service.
If you want to learn more about Influencer Marketing, here is a post I did last year: "Influencer Marketing: The Next Level of Content Marketing" https://www.linkedin.com/pulse/influencer-marketing-next-level-content-shane-barker
I will also be coming out with an Influencer ebook very soon that will help shed some light on the subject. So stay tuned. ;)