Maybe something like….
Freelance Brand Consultant – Collaborated with companies to produce and distribute content for short-term advertising campaigns in the [your niche] space. I worked with over [number] of brands including most notably X, Y, and Z. [Add some impressive stats about traffic that you funneled to them if you have any to add].
I would shy away from mentioning your specific number of followers unless the job you were applying for was in the same niche, and you could use it for proven experience in the niche or if it became such an impressive number that people might hire you because they think your name would mean something to brands they are working with from a general perspective rather than a niche one (but I think that would only apply if you had millions of followers instead of tens of thousands).
As general advice to people reading this, I wouldn’t necessarily include this information if it was not a job that was tangentially related that you are applying to, either it being in social marketing or it being in the same niche (say you are applying for a software engineering job and have a hugely popular social presence related to coding and it could boost the company just for you to list them as your employer). Or alternately, if you don’t have anything else to list on your resume for that time and it would otherwise create an employment gap to not have anything.