I'd be careful with a free hosting service
you will need:
1) Volunteers to handle the sign ups and approving/disapprove
2) High conversion ratio to paid upgrades to generate income
3) Passive ad income to at least cover the servers costs
4) High level of security, mainly phishing protection and email protection since free accounts are notorious for warez files, phishing, mass spam, fraud, bots, viruses and much more.
5) Have some form of automated bot sign-up protection to prevent mass sign ups