Advertising companies like Facebook make their revenue by selling to advertisers, not individuals- they are emphatically a B2B company.
I would argue that this is not totaly true. Everyone can advertise on Facebook (I did it myself without a company).
They're both. They're selling a service to consumers, which they buy with their personal data. They then use that data to sell to advertisers in the form of better targeted ads.
They didn't even run ads for the first three years of their existence.
Are you saying they are bartering and not paying sales tax for their services and their customers personal data ?
Probably because there is money to be made from advertising/sharing user data and human customers don't have the money to see you like businesses.