When ur best friend spends more time with her other friends...

There will come a time when you will spend less value to the amount of time you spend with friends. Because, and this is purely the way I see it, true friends don't need regular contact. It can take days, weeks you don't talk with each other and then when you do, you just continue your talks about the subjects from when you talked together the last time. As a teenager I think it is logical to have more need to frequent contact with friends, but as you'll get older and you'll get a busier life with work, household, relationships,.. you'll be happy enough to just once in a while have someone to talk with.

So, now to answer your actual question, all you can do is ask that friend why he / she spends more time with their friends, if it is a personal thing or a wrong impression. Like "Hey uhm I was just wandering... I feel like you're spending less time with our friendship than with your friends. Is this merely an impression or.. ? I just like to know before I jump to conclusions too quickly.."