I have a few "friends". Some are in Poly relationships that are fully open. Works perfectly for me.
one is a long term FWB. We go for nights out occasionally. But meet up when suits us both.
two are Poly so have existing relationships which is fab. The wife's/partners get day to day stuff and I get easy going fun. (Fully open, wife/partner also have other relationships).
A possible date next week for another one too.
I see them very flexibly. So maybe once a month or something. All very mood dependent on both parties.
A couple of things though. I definitely don't want a relationship, so I'm not on OLD other than for FWB. My profile clearly states that. Be careful you aren't just settling for FWB, or hoping it will turn into something more. You'll tie yourself in knots for something that won't happen.
FWB means you can't really be a jealous type of person. They can, and likely Will be seeing other people. For me, I'm very much enjoy. So long as when we're together we're practising safe sex and I've got their undivided attention in that time frame it's fine for me. I've had friends that have tried, and hated FWB because of the aspect of the other person seeing others too.
if you want FWB but exclusive you are just dressing it up differently but want a relationship. Again be careful with this. Because they can simply not turn up one day, you'll have invested yourself into it, and they'll say it "was only FWB".
Good luck, it's not for everyone, but I do enjoy my life as is. It doesn't have to be forever either. I've had friends do FWB, then update all profiles as they are now ready to settle down and look for something else.