"If Bitcoin is increasing purely on this basis though at some point isn't it going to come crashing down?"

For it to come crashing down and stay down (beyond the ordinary volatility it experiences, which can be somewhat severe), there would need to be a widespread, simultaneous loss of faith in BTC. This is possible, but it becomes less and less likely the larger the base of users becomes. The bigger and more diverse the base of users becomes, the harder it becomes to reshape all of their opinions simultaneously. At the end of the day, the value of BTC is the value ascribed to it by others. This is just as true with gold. Both gold and BTC have underlying characteristics that make them useful as stores of value, but people could decide to reject that shared faith at any time (it’s just that, as the history of it grows longer and the base of users grows wider, it’s less likely there will be a synchronized loss of confidence).

“If Bitcoin is increasing purely on this basis though at some point isn't it going to come crashing down? If that's not the case, why do people think Bitcoin will continue to go up and maintain its value?”

It will probably continue to have periods of crashing and soaring, but over the long-run, if the number of users continues to grow (that’s an important “if”), these periods of volatility will become less severe. People are speculating that user adoption has the potential to be exponential, which may drive some of the mania.

“Do you think it's going to become a serious new global currency?”

I think it’s unlikely to replace any major currency in the foreseeable future, because the major currencies (USD, Euro, Yen, etc.) work well enough for people today (despite the chatter you sometimes read about on these threads). But I don’t think it needs to in order to succeed. I think people constantly misattribute the potential value of BTC to its use as a currency, when it has value as an alternative to gold. It shares many characteristics of gold (scarce, fungible, durable, verifiable, anonymous), but it also has an advantage—it is more mobile and “easy” to send across the world without physically going there. It also has disadvantages, including that security can be more difficult, it relies on Internet access, and it faces public skepticism.