They weren't ever your friends. People at work rarely transcend beyond co-workers and into actual friendship. If all you do together is work and around-work activities, they probably aren't your friends. You're effectively people locked in a cage together that happen to get along.
If you get together on days you don't work together then they might actually be your friend. If they've met your non-work friends they might be your friend. If they attend your birthday party where they don't work with 95% of people there they might be your friend.
If you don't invite them to those things you don't actually consider them a friend. If you hold any power over someone (e.g. CTO-employee relationships) and they do this stuff they still might not be your friend. But if they don't do any of this stuff they definitely, 100% are not your friend.
Also, one of the best ways to fuck up a friendship is to hire them. You need to be sure you can properly separate business and friendship if you ever do this.