Oh, boy, well, if it makes you feel any better, I've been in a similar situation. You see, I've had "online" girlfriends, four to be exact. Now, if I remember correctly, it went Zyn -> Shira -> Irene -> Myuu. Now, I've always loved Zyn, she was the best. I don't exactly remember how we broke up, but I'm leaning towards lack of communication for a long period of time. I was starting a relationship with Shira, I liked Shira, she was cool, but just when I was about to plunge into that relationship, Zyn said something along the lines of, "I love you," I can't remember exactly, I just know "love" was used. You see, we started talking again recently, just out of the blue conversations, and Zyn and I still still chat from time-to-time, but you know...as much as I loved Zyn, and as much as I would have loved to start that relationship over again, I also wanted to try something new.
So...despite having a huge argument with myself, and getting super emotional with Zyn in a conversation that same night...I chose Shira. I don't regret it, though sometimes I wonder what would have happened if I went the other direction, but hell...this was two years ago, I was 16/17, and I was having one of my moments to try something different, to see how something would go.
Then...around a month or so later, Shira and I broke up.
You see, I'm not telling you to break up with your boyfriend, and I'm not telling you to start a loving relationship with the person who has a crush on you, just be aware of what you'll put yourself through with either direction you take. Making a decision is tough, and there will be times that you regret that decision, especially when you've been with someone for...a year and a half. (Longer then me by the way, I was with Zyn for...I want to say a little more then half a year. Shira was a month or two, Irene was a month, and Myuu was two.) It's alright to feel happy that someone has a crush on you. Because that's perfectly normal, don't feel like you're betraying your boyfriend in any way. It's not everyday someone says they have a crush on you, so embrace it. Then, depending on what type of boyfriend you have, share it with him, and maybe even ask his advice regarding how to handle the situation. But...if he's the kind that gets jealous rather easily, I wouldn't tell him. That would just cause trouble.
Don't feel ashamed. You don't have a reason to be.