I suspect that what you are experiencing is very common. I'm a recent convert and am experiencing what you described full force at the moment, so you are not alone! Converting, or becoming more serious about your faith (which is a conversion as well, I suppose), has been the beginning of what I call "the great purgation" in my life. Losing friends seems to be one of the many things that has accompanied my conversion that has caused much pain.
I was very involved in my Protestant circle. I was quite the social butterfly. Hardly a day went by without me hanging out with a group of friends or spending one on one time with friends. My recently received and dialed list on my phone had, on any given day, at least 8 or 9 calls from different people. Now, I'm lucky if I get one phone call a day... and its usually my mom. Lol. I have only one good, solid, catholic friend; he happens to be the one who converted me. And we don't speak that often anymore, since he's a guy and I'm a gal. The transition has been really hard, and lonliness is definately something that creeps up on me. I've had a hard time meeting people at my parish, especially young single folks that I can be friends with.
The few Protestant friends that made it through my conversion with me are wonderful and I love them dearly, but there is still much strain on our relationships due to doctrinal differences. Because of this, communication becomes less, and the friendship tie continues to weaken.
If you are anything like me, becoming more serious about your faith has included a big lifestyle overhaul. My conversion completely switched my way of living life. The way I talk with God, the way I worship, the activities that I participate in, the kind of stuff that entertains me... these have all changed based upon my discovery of good Catholic teaching and culture. Its not that I suddenly started disliking my friends, its that we suddenly had very little in common anymore.
Spending less time with friends left a great opportunity for intimacy with God. I now spend a great deal of time in prayer, doing spiritual reading, listening to sermons, etc. I do things like writing letters now. I didn't have time before. I think this friend depletion in a way was God getting my attention and letting me know I had been neglecting him. My advice is to use this time to develop a strong friendship with Our Lord.
Being a largely sanguine personality, I need friendship though. That is why I started cruising traditional Catholic forums. I started posting more and reading others posts. Eventually I got brave and private messaged a couple people whom I either respected, found hilarious or entertaining, felt a connection to or all of the above, and got to know them better. Suddenly those names on a screen became real life friends, and that has been absolutely wonderful. I hope to meet them in person at some point in the future. If you don't have a screenname already, maybe you should get one and begin building some relationships with some people here or on another forum.
An ave for you.
Blessings to you, anonymous. :)