Is it permissible to deal with free or paid service companies — such as email, website, and internet services — that promise specific storage spaces or speeds, knowing that these companies cannot provide these services to all subscribers simultaneously, which makes their promises include a form of gharar (ambiguity/uncertainty)?

The Gharar (uncertainty) in the free email service does not affect it because it is a gift, and Gharar in gratuitous contracts is forgiven. As for the paid subscription, it is a form of ijara (leasing/hiring). The occasional unavailability of the service is a minor, excusable Gharar. However, if the company does not own the service or is unable to provide it, that would invalidate the contract.