The medium isn't the first question -- it is who is the target audience

Years ago Harry Webber wrote "Divide and Conquer" which broke ground for segmenting and targeting an audience.

The key to all of this is finding the correct medium to reach your target audience, be it traditional, social, or digital mediums.

Integrated Brand Promotion uses all mediums, advertising, promotions, and public relations to create a synergy otherwise not possible with a single medium or discipline.

Astute marketers and advertisers find agencies that can build their brand(s), affect a change in their target consumer and within their competitive category with a resulting increase in sales, volume and profits. Obviously, the more effective the consumer targeting is, the greater the impact and results for the advertiser.

Remember, advertising, public relations and promotions are arts, not science. Mediums say they are a science because of their heavy reliance on quantitative techniques, but the art comes in when they qualitatively compete among themselves for audience share. 

Also note that we are on the cusp of a great change in communications that has seen dramatic change over just this past decade. The winners will be those who understand how to impact their target audience within their competitive category, be it marketer, advertiser, or advertising agency.

The definition of Advertising is: a paid, mass-mediated attempt to persuade. Public Relations is: a non-paid, mass-mediated attempt to persuade. When an advertiser or advertising agency pays to have content place on a social media website -- that is Advertising as it is paid. When content from an advertiser or advertising agency is place on a social media site that is not paid for it is Public Relations.

Public Relations is best practiced by PR Agencies - which is why advertisers hire them to influence, create good will, and change opinions and attitudes (or form them) about brands. Only PR Agencies can deal with Crisis Management. There have been many well documented case studies where ad agencies attempted viral or social media strategies that have completely backfire causing harm to brands -- that is why it is best to hire agencies with strong public relations backgrounds for social media efforts.