When was the last time you saw a business with nothing but an Instagram profile, or saw Instagram referenced on the news, or cited in a paper, or used for a press release?
It's not about active users, it's about how it's used.
Are you saying AOL wasn't mainstream? The Internet might have been niche as a whole, but out of that, AOL was as mainstream as it got. AOL was synonymous with Internet, like Facebook and Twitter are synonymous with social media.
I don't think that's a fair comparison. AOL didn't initially position itself as a portal. It positioned itself as an ISP with a portal.
Had AOL built itself as a web portal like Yahoo from the start it may very well have stayed relevant. I still have business partners (in non tech fields) that use their AOL email and news/portal services.
It's not about active users, it's about how it's used.