Some additions to the other answer:
Pinging either of them echoes back from an ip address of 198.35.26.96. So whatever PC's these two websites are hosted on, they are connected to the internet by a common router, which is then port-forwarded to either of these PC's, right?
In general, what you describe is possible, but most websites have grown a bit beyond that – very little about home/smb internet providers applies to large-scale hosting. (Compare e.g. a mp3 player and a professional recording studio.)
In the specific case of Wikimedia it's not just one PC per site anymore – it's several racks of servers storing the articles, the images, the wiki code, etc. (See their own webpage about that, or photos and blog posts on Google.) Chances are that Wikimedia's network is bigger than some ISPs.
But even though they have many servers, it's usually not a one-site-per-server setup – often a single server hosts many websites, and one IP address might be load-balanced between many servers. (The "many websites on one server" part is usually called virtual hosting. All HTTP requests include a Host: header telling the server which site you tried to access. That makes it possible to host even hundreds or thousands of sites on the same server, same address, same port – the HTTP service just looks up the matching configuration.)
I didn't know Wikimedia Foundation would count as an ISP; I thought the ISP would be some other networking firm with a wireless tower here and there(you got the idea), who connects wikipedia with the world with a strong broadband connection.
Taken literally, no, Wikimedia is not an Internet service provider. But such websites use the term 'ISP' rather broadly – e.g. in this case they probably mean an 'autonomous system' owner, which might be the Internet equivalent of a legal entity – you need to have an AS number to be able to own IP addresses and announce them to the rest of the Internet using BGP. Since most autonomous systems are ISPs, some websites just use the terms interchangeably.
That doesn't mean they're the same thing though; anyone (even an individual) can obtain an AS number after some paperwork. For example, many organizations do this as part of setting up a redundant connection – they obtain their own ASN, buy a block of IP addresses, and announce it through both ISPs at once – in essence, becoming part of the Internet exactly like ISPs themselves do.
Also the IP host is a subdomain of wikimedia.org, which is equally puzzling. How could wikimedia.org be defined and registered even before the IP host had been defined and registered?
Domain registration is entirely separate from IP addresses – buying a domain merely reserves the name, and then you configure where it should point. (Most domain registrars have a control panel where you can edit the domain and subdomains.)
Similarly, reverse DNS (mapping an IP address back to some domain name) can be changed at any time by the IP's owner.
