Currently Bitcoin have +10 millions users, according to Blockchain wallet info. This is very small part of whole world market, so in fact Bitcoin have very small niche, but Bitcoin can expand in future, if large web sites, like Amazon will start to use it.
And 90% of those wallets can be owned by just a couple of people who are running big websites. People can create unlimited number of wallets so those numbers mean nothing at all. It can serve as the upper limit but the actual numbers are really far from that. We're just about a million, more or less. But we keep on increasing in popularity as day goes by since some people are making good money from the bitcoin world and can't keep talking about it to their friends.
You can already purchase things from Amazon with bitcoins using purse.io. It's a third party so it's not really Amazon that accepts bitcoin.