Influencer

An influencer is someone who has:

• the power to affect the purchasing decisions of others because of their authority, knowledge, position, or relationship with their audience.
• a following in a distinct niche, with whom they actively engage. The size of the following depends on the size of their chosen niche.

As one particularly honest marketing blog (which I will not link to) puts it:

It is important to note that these individuals are not merely marketing tools, but rather social relationship assets with which brands can collaborate to achieve their marketing objectives. (emphasis mine)