This is the key argument for people promoting Bitcoin: Look how where we were a few months ago and look where we are now. It is only going faster - you should join.
Anyone that uses that reasoning doesn't understand the fundamentals of what bitcoins is, how it works, what it's benefits are, and what its potential is.
This means that these people have jumped out of an airplane with no parachute.
If you exchange anything of value for bitcoins and you are only doing it because "it's going up and it might keep going up", then you are a fool. If you have more value in bitcoin than you are comfortable losing, then you are an even bigger fool.
It's not quite the same as "jumping out of an airplane with no parachute", because Bitcoin actually has a lot of potential for success whereas jumping out of an airplane without a parachute has almost no potential for success. It is more like being handed a parachute pack and being told that there is a parachute in there, then jumping out of the airplane without first checking that there actually is a parachute, or that it is in good condition.
They are now halfway down and are telling their friends: Look how well this is working, and it is going faster and faster.
Correct. And if it the plane is on fire and it turns out that there actually is a parachute in the pack that they were given, then they are going to be VERY happy that they trusted what they were told and jumped. On the other hand if the plane lands safely, and the pack doesn't have a parachute, then they are going to regret their decision. Until they pull that rip-cord, they'll never know which it is.
When you tell the people they are going to crash-land, they just tell you it is FUD and that in their experience things have been going great since they jumped out of the airplane.
Correct. Only time will tell if there actually is a parachute in that pack they put on. There's a good chance they'll be fine, but there is a very real possibility that they will regret their decision.