I'm going to make this easy… If your BFF has, or wants to have a boyfriend (or girlfriend) dump her. Because she's going to dump you. Trust me, she's a BFFB. And you're an emotional seat-filler in the Academy Awards of life and the second she gets that shiny trophy, you're on the fast track to the C-list, standing on the other side of the velvet rope. C-list? Yep. B-list is reserved for girls who know the new love of your friend's life, because it's SOOOO much fun spending time with them and finding out all kinds of hysterical things about the new trophy. What you'll get are conversational table scraps. These are topics that the trophy isn't interested in talking about. Shoes, hair, Gossip Girl, etc. Remember all those nights you and your BFF stayed up talking about sex? Now the only sex you'll get to hear about is trophy sex. And if you think having sex with the same person night after night after night is mind numbingly boring, wait till you have to hear about it day after day. The most time you'll ever get to spend with your friend now is when she's in a fight with the trophy. Or wants to piss the trophy off. And if you ever try to point out how it's been a while since you had a "girl's night out", or got a mani-pedi together, and how long ago you both swore that no matter what happened, you'd never let a boyfriend or girlfriend come between you…you'll get accused of being "jealous", or "needy". That's a BFFB.
I agree with Catherine on part of what she's saying…if your BFF is in a same sex relationship – forget it. She's definitely a BFFB. You just can't compete. That new girlfriend can be your BFF's lover, friend, sister, everything. So back off, hope for an invite to the occasional dinner party and maybe if they live in California or Canada you'll be a bridesmaid at the wedding. With straight girls it's different. Not all BFF's will dump you for the boyfriend. Only the stupid ones. The ones who say things like, "We're one person", or "He knows me better than anyone", or worst of all "He's my best friend". You can never be best friends with a guy. That's not what they're built for. They're built to have sex with. And maybe a handful of other things – all of which you can hire someone to do better and faster. And when you hire a handyman to fix your mailbox, you don't have to stroke their precious ego for an hour after he does a half-assed job. Men are different than women. And that's okay. So long as you understand it and act accordingly. Stupid women spend hours and days and months and years trying to teach their boyfriends/husbands to communicate better. Give it up. They suck at communicating. And all the time you waste trying to change them, you could be actually communicating with people who love it…your girlfriends. Men don't want to talk about their feelings, or go shopping, or trash people about their horrific fashion choices. They want to have sex and eat and watch sports. Great. The first two things you don't want to do with your girlfriends anyway, so these are perfectly acceptable boyfriend activities (but remind me to tell you some stories about a few great exceptions to the first rule). Sports he can watch by himself or with his BFF's (yes, they have them too). That leaves EVERYTHING else for you to enjoy with your girls. And if you understand that, you'll never be a BFFB.