I have to agree with jrm4 - all the things you pointed out don't explain why FB groups and markets are so popular. There is also bunch of businesses that don't have their own website, just FB page - which is not comparable to "giving people simple text box".

It is simplified web experience from point of business owner, just drop logo, type in your company name and you have web presence - which happens to be where people are because they had friends/family there anyway.