Why does my ex girlfriend's best friend stalk me online?

Long story short, my ex dumped me and cut off contact months ago. I initially disturbed her a lot, trying to reach out to her but she didn't respond. I surprisingly talked everyday to her best friend and even she got pissed off and told. me to stay away from her.
2 months later I wrote a text and apologized for the way I behaved and ended on a goodbye, and she finally replied so I sorta got the closure. I know I wasn't exactly at fault, everyone said I was the nice guy, and she replied because I sorta blamed myself in the text but I had to make peace with myself.
She did sorry for the way it ended. The thing is, that best friend of hers? She stalks me online quite frequently, and it initially didn't matter. But then it just sorta increased. I know it's not my ex, because they don't share each other's passwords, but why is her best friend so keen on knowing everything in my life. We don't even follow each other. And initially I was blocked by her too during which she would unblock me and watch my story and then block me. again.
Why is she stalking me so much?
Girls, have a clue?
Why does my ex girlfriend's best friend stalk me online?
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