If there was no promotion before, first focus on the performance. And when social networks reach a stable and scalable sales volume - increase the work on branding.
If SMM has already worked on one of the goals, and, importantly, worked well, add more work on the second goal. And if the efficiency was low, you need to change the approach according to the scheme above.
Regardless of the fact that the final goal is sales, you need to understand that in each project SMM, as a channel, can be assigned a separate role, as a piece of the general marketing funnel.
And these are not always sales. Sometimes SMM is present as a tool for primary audience coverage. Then you need to choose a KPI based on the coverage indicators. For example: CPM, audience coverage, etc.
Or if the goal in SMM is communication with the audience, then you can take the reaction indicators, ER, ERReach, etc. as KPIs.
SMM as a channel may not bring final sales or leads, but this does not mean that it does not affect the overall result. In any case, you should proceed from the overall strategy of the project; you cannot do SMM in isolation from the rest of the marketing.