  Pat, Why a best friend of 9 years choose her Boyfriend over me? Tough one, it's hard to say without knowing the people involved.
But it reminds me of when I got in my first real mature relationship and got me a girlfriend, whom later became my wife. My friends were very protective of me and wanted to make sure that she wasn't going to mess with my head... too much. Also it took a while for them to warm up to her. And yes I did spend less time with them as I spent more time with her, which was difficult for me and probably my friends.
But you only have so much free time and each person has to decide their own priorities. Eventually my friends realized that I was serious about this girl and they understood why I was with her "all the time". Like I said though, in my situation I knew my relationship was serious and it was worth investing the time into it.
I don't know your situation enough. But if it's a serious thing then I'd say she'll make time to hang with you but it might be less than before. But if this is just some fling for her then she's definately blowing you off, which is not cool. If this is the situation, then I bet the guy is either pampering her or is good in bed or something like that. I need more intel on this one for further diagnosis. 
