I really don't know if I'm overthinking this or not. I went out with a friend last night to an event - she knew a fair few people that were attending but I didn't know anyone. As we were mingling my friend was greeting and chatting to various people with me by her side but not once did she introduce me any of them. For a big part of the evening I was asking her 'Who was that?' and feeling like a complete spare part (and a dork).
I'm not shy or socially awkward but I find it easier to chat to strangers if I know their name or something about them. At one point I was feeling so pissed off and isolated I asked my friend to introduce me to people and she just gave me a blank look 
I don't expect a whole Bridget Jones 'This is Perpetua' speech but surely most normal people introduce their friends to complete strangers?