the main focus of your question depends on who in the phrase 'the strategies will differ depending upon who they are seeking to communicate with. '
whom, indeed , is a more accurate description to 'who'.
in this phrase whom means the customers.
I can rephrase the sentence' maybe concentrate upon a certain sector (retail companies for example) as the strategies will differ depending upon who they are seeking to communicate with. ' into
'maybe concentrate more on a certain sector (retail companies for example) asthey may use different communcation strategies depending on the types of customers.