Social media's impact on buying decisions is growing rapidly but its influence varies across product categories, according to the McKinsey Quarterly.

add to cartThe management consultant surveyed purchase decisions of 20,000 European consumers and more than 100 brands.

It found social media recommendations influenced 26 percent purchases across the 30 surveyed product categories. Direct recommendations played a role in two-thirds of the purchases.

Recommendations were most sought out by consumers in the travel, over-the-counter drugs and investments categories. Utilities ranked lowest.

The consultant found that a small group of "power influencers" accounted for nearly a quarter of total recommendations.

For companies looking to maximize return on social media, McKinsey says they should "both encourage would-be customers to engage in more social interactions and inspire more influencers to express enthusiasm for their products."

The research is bullish on the role of PR and third-party endorsements. It found that online articles posted by journalists encourage people to gather more social input to guide purchase decisions.

According to McKinsey: "Public relations spending to generate such articles may be a worthwhile investment."

Spending effectively on search-engine optimization to move products up the search results can expect to benefit from a greater social-media impact.

McKinsey also found some companies demonstrated that they could turn negative vibes [from negative feedback) to their advantage by responding quickly, according to the "Getting a Sharper Picture on Social Media's Influence" written by Jacques Bughin.