I know IABU, clearly she's entitled to be friends with anyone she likes, but I still feel a bit gutted and like I'm being replaced.
She talks about the new friend a lot - about how lovely she is (she is, that's true) and mentions how she's been round to see her, and even sends me screen shots of bits of their chats. 
Previously my friend was very rarely on messenger and I was pretty much the only person she messaged with it, but now I can see my friend and her new friend are both active at the same time A LOT (I know the new friend as an acquaintance which is why she shows up on my messenger app).
As background, my friend and I went through a really horrible situation together and it bonded us. We've spent the last few years not inseparable as such - we both have two kids and busy lives - but really close.
For the record, I know it's really a silly thing, I'm not stalking her, I know she can make new friends etc. We're both 40-something women, not teenagers, so I'm not looking to create drama or anything like that. I've just had a few friendship issues with some of my oldest friends so I guess I'm feeling extra sensitive at the moment.
So AIBU to ask you for your tales of similar situations and how you dealt with it?