I suppose it would be if you two have never met in person before. But since you have I don’t see anything weird about it.
I do online dating, I know women like to talk on the phone for a little first before meeting up to see if their freak radar pings since we’ve never met, as I want to talk to them as well before meeting. But since you’ve two have already met you’ve likely past the freak test.
Just send a message like “Hey what you doing friday night ? Want to go out for dinner/ museum ?” Leave your number at the end of the message.
But then I’m an old dude who’s never had a FB account.
Go with what you feel is right for you.
Hope it works out .