Most of the features you're talking about existed before FB in some way. FB doesn't hold a monopoly on introductions or communication and we did just fine before it.
Your argument that you couldn't swap numbers with your fiancé doesn't hold water with me. In a world without FB, it wouldn't occur to you that somebody you didn't swap numbers with would be your partner. Instead, you'd be marrying somebody you did swap numbers with and be none the wiser.
I realise re-reading my comment that this example wasn’t clear. Sorry about that.
I’ve dealt with dating and meeting people at parties before and after Facebook took over my social circle. Facebook meaningfully improved the experience for me, mainly through the two avowed goals of the platform: universality and “real” identity (I hate the word ‘real’ but it’s the most legible way to combine recognisable face photos, civil name, personal details and representative social graph). Yes, you can ask for a phone number, but that’s a rather transparent ask; the person you are asking it from usually doesn’t have enough context to know if they want to expose themselves that much, lead me to believe there is a chance, etc. In my case, I know for a fact that, if I had asked for it, she would have preferred to give it to me but would have not (she was in the process of breaking up and felt this was too early; she was fully single couple of days later, when I wrote to her).
During the earlier stages of dating, I believe that a ambiguity helps (and that is very much something that my American colleagues at Facebook disagreed with). I believe that by reaching out by being helpful rather than assertive, you can offer a less macho version of masculinity. Then again: if you knew me, you’d know I’m not good at this, at all. ‘Sliding in your DMs’ as it became known is not entirely positive, but it does take away some responsibility from a party that is not willing to appear keen. Male friends more attractive than I am have also noticed that it offers more initiative to female daters -- whether that’s the technology or the time, I can’t tell. It also allows you to hide or block unsuccessful attempts in a way that many social circles don’t.
One thing that is my experience and that is universal is that having a social graph helps greatly to pick partners: you have common friends you can rely on to insure the person is reliable, more context, things to talk about. Dating apps have taken those features in stride (and still have access to those, i.e. meaningfully more information than other apps have on the Facebook API). That does help compared to the stranger-in-a-bar deal. It might foster homophily, more superficial matching, etc., once again: I don’t know of an exhaustive study on the subject. However, I do know from working directly with that team that Facebook cares an is willing to build tools to help, and support companies trying to do the same.
I realise that my experience is anecdotic. That was my original point: a long list of anecdotic, positive experiences through Facebook. In that anecdote, I did find the courage to ask someone for their phone number, a while earlier, I did end up with someone else. She just wasn’t nearly as amazing, and that’s why I was hanging out, sad and single at that party. Had I known more about the first one, say, through Facebook, I would probably have realised faster that we were not meant for each other.
Your argument that you couldn't swap numbers with your fiancé doesn't hold water with me. In a world without FB, it wouldn't occur to you that somebody you didn't swap numbers with would be your partner. Instead, you'd be marrying somebody you did swap numbers with and be none the wiser.