Congratulations - that's bound to be a lively and interesting niche.
My take is that the differences in scope of these two niches is quite clear - yours having a more narrow focus on marketing, and the other one being all about social media in general. This is a classic category-subcategory configuration that we expect to work well. Different users will adopt different posting patterns depending on how narrow their focus is - the laser-focused ones will only post their social media marketing content to your niche, while the others will probably post it to both. As far as consumers go, the same pattern will apply perhaps more strongly: if I'm interested in social media marketing but not in social media games, chances are I will only subscribe to your niche so I can avoid the 'noise' from the more general one.
Please keep checking in with progress from time to time, once Beta launches and your niche develops: I'm interested in following your journey with this one!