There's a lot of fud going around and a lot of it is coming from naysayers who are butthurt they missed out on the bitcoin boom. However, that's not to say bitcoin isn't going to be a bubble, but as the above guy said, it depends on how you define a bubble. If bitcoin falls in value dramatically for whatever reason then people will say the bubble burst and feel pretty smug with themselves. However, of course it could keep going up and up but I don't think that can last forever and if you look at bitcoin's history it is full of many up and downs and I'm sure both of those things will happen again in the near future.