I had the same problem but this was with one of my female best friend. We was good friends for the 8 years we had known each other, go out for lunches, clubbing, cinema, I even bedded some of her mates
Anyway, first time she got a serious boyfriend she was ok for a few months and contact was gone. No answer to phone calls, no reply to text message and emails. I was worried so I went to her house, got speaking to her parents and they said she had moved in with her boyfriend. So I left it as that as I was peed she had completely ignored me. Four months after I went to her house again, parents explain she has broken up with him and lived at her nans, during this time her number had change and was given to me by her parents to call her. So i did, surprise surprise, she answered and we got talking. It was like nothing had happened. 8 months later, she finds another person and history repeats itself. Its 18 months since I last had contact with her, went to her parents house back in August, just incase something did happen and she is all fine and danny living with her current boyfriend. Again, no answer to phone calls, no reply to text message and emails.
I have 3 best males friends, one who I have known for 16 years, one for 10 years and the other for 13 years. We have never broken contact with each other and I see them nearly every week without fail. And if we don't, that phone gets used instead. Keeping in mind 2 of them are married with kids and contact still hasn't broken.
So my opinion is, if they have the ignorance and disrespect to drop in and out of your friendship when it suites them then ditch them. They ain't worth the hassle. You don't do it to them, so you expect them not to do it to you.
Boyfriends and girlfriends will come and go but friends will always be there. For those so called friends who have a bad habit of ditching their friends every time they find someone new, then they are going to grow up to be a extremely lonely person.