Lets correct a few things first of all: There are no current plans to change WhatsApp, merge it with Facebook Messenger, add adverts etc. Much like when Facebook bought Instagram this didn’t change how that app functioned. This is quoted in several of the better news reports on the story and is on WhatsApp’s own blog.
WhatsApp is an ‘instant messenger app’ or SMS (Texting) replacement. Yes you can send a text through normal means but as mentioned that often requires a phone signal and can be costly to do abroad. There are no additional charges for this other than data, so it is free if you’re on Wifi etc.
It is also very useful for groups and group conversations where texts are not, not only is it cross platform so your Windows, Android, iOS, Blackberry friends can all chat together but it also allows you to mute a conversation, add a new person, remove a person etc. nothing that can be done on SMS.
People have mentioned sharing media which it is also very good for, much quicker than by SMS and lets you share more than just pictures, you also get a delivery verification, you can setup unique tones etc.
The chart of its growth where it surpasses Facebook’s user base is phenomenal considering it isn’t even that popular in America yet according to the reports. Clearly Facebook sees some potential in the investment or just want to get hold of the fastest growing communication tool going.