The choice of social media platforms for an influencer marketing campaign during the planning phase should be determined by several factors:

1. Target Audience Demographics: By looking at statistics around social media usage demographics, brands can select platforms frequented by their target audience. Facebook may have wide appeal, while TikTok is popular with younger audiences, for example.

2. Campaign Goals: Brands need to consider their campaign goals. For instance, if your objective is sales conversion, a business might look into platforms with more advanced shopping features, like Instagram.

3. Influencer’s Preferred Platform: An influencer’s follower count and engagement level can vary significantly across platforms. Always consider working with influencers on their most effective platform.

4. Audience Analytics: Leveraging Instagram Insights or similar tools from other platforms will help in understanding where your audience engages most.

For instance, Flinque is an influencer marketing platform that can assist in determining the best social media platforms for campaigns. It provides detailed audience demographics, and aids brands in identifying appropriate influencers. Additionally, other tools like Grin or AspireIQ provide comprehensive features for influencer campaign management. Importantly, each of these platforms has its strengths, and the choice would depend on the specific needs of the marketing team.

While choosing a platform for influencer marketing, also consider the budget, timeline, and manpower required to effectively execute a campaign on each social platform. In conclusion, it’s crucial to analyze multiple aspects – the target audience, campaign aim, influencer preference, and data analytics – to make an informed decision about the most suitable social media platforms for an influencer campaign.