Whatsapp is already one of the most popular ways to communicate with friends on mobile devices, especially when you're talking cross-platform communication. Today they're announcing that they've reached a new milestone of 400 million active users (meaning the users are using it at least once every month), which is up 100 million from just 4 months ago.
This is very impressive growth for a new chat company, in the days of Hangouts and Skype and iMessage, which are all owned by major companies now. However, the chat ecosystem seems to be always reshuffling and evolving, with new players coming into the market and becoming popular every few years or so, one of which seems to be Snapchat, too.
Just like with Snapchat, there were some rumors earlier this year that Google wants to buy Whatsapp for about $1 billion, but it seems the company turned them down. I think it would've been worth it for Google to pay even twice as much, considering they wanted to pay four times that amount for Snapchat, which has ten times fewer users (although with higher engagement).
On the other hand, it's probably for the best to not have 2-3 companies owning everything in the tech world, and it's better to have platforms that are truly cross-platform, and don't have any conflicts of interests with other platforms.
The thing that I'd criticize about Whatsapp, is that so far they haven't taken security too seriously. There have been quite a few security issues with Whatsapp in the past, with the recent one happening only two months ago. Governments and other hackers alike, are going to try and obtain that data one way or another, now that Whatsapp has a target painted on their backs, because they have so many users. As long as the data is only secured in-transport, that can be easily intercepted, or Whatspp can be forced to intercept it, so it really doesn't matter whether they delete it from their servers later or not.
I'd like to see them implement end-to-end security, much like TextSecure, which has recently been integrated into Cyanogen, and offer their users true privacy and security (even more so than the "privacy app" Snapchat, which seems to be all the rage these days). Snapchat pales in comparison with TextSecure when it comes to privacy and security, and it's actually the gold standard in secure messaging today.
The good news is that Whatsapp doesn't even have to replicate its functionality from scratch. TextSecure and its protocol are fully open source, and all they need to do is implement it. Perhaps with a few requests from their users, they may actually do it.