That is a tough one! I would go with instagram.
I enjoy it because it is a short video or a picture with some text, while allowing you to look into someone else’s life or interests makes it kind of exciting. It might also be because I love the photography aspect of it all.
Of course, it probably dates me, but Facebook. It, along with Youtube, continues to have the largest demographic of users than any other social media platform. Users range from teens to 65+ (my 83 year old grandmom is on the Book), and the fact that Facebook purchased Instagram means that when integrated you can still post and reach a vast audience of young adults who are on Instagram as well. Facebook ad campaigns can expand to Instagram, thus reaching both audiences of people.
I am so with you Sommer! I continue to dabble in the others, but FB remains at the top of the list for me. Just have to stay current with how they change it up! I agree with you that we are still able to reach more of the demographic via FB as people of all ages are now using it. Instagram is a close 2nd, but I am still getting up to speed on it.
And, it is best to focus on just 1 or 2 platforms (maybe 3 at the most)! Pick your favorites and stick with it. Consistency is key!!