Lately several post have led me to believe that some members think WhatsApp is some sort of dating and / or social media app where you can 'find' chicas - like Tinder and Badoo (or even as guys use Facebook in Colombia).
First, what is "WhatsApp"? It is an end-to-end encrypted messaging phone (primarily) app. Messaging.
You use it to send text messages (chat) or make voice (or even video) calls over both WiFi and cellular data networks. It is the single most popular messaging app in the world and everyone in Costa Rica uses it.
Why not just call? Because that uses talk minutes. Why not just use SMS? Because that uses SMS time. So what's the advantage? WiFi.
Costa Ricans use WhatsApp, primarily over WiFi, because many have pay-as-you-go cell plans and this gives them a way to text and call without using their cell carrier and using up their data plan / minutes / SMS. Often people have no $ on their phone, but they are in a WiFi hotspot - with WhatsApp they can still text and call. This is true in Colombia, Peru, Mexico, etc.
Can you search WhatsApp and find girls? No. It isn't 'social media' or a dating site. You can send a WhatsApp message (or a make WhatsApp call) to
someone in your contacts that has WhatsApp.
So why should you use it?
So, you meet a chica and the session is awesome. You want to do it again tomorrow. You get her phone number, put it in your phone and then send her a WhatsApp. And since it's over WiFi, it will work anywhere there is WiFi (I can WhatsApp to Costa Rica from the US on my CR phone with a Kolbi SIM that doesn't work in the US - free) so you can send her a WhatsApp before your next trip. Or wander all over the Gulch looking for her.
Why not just call? So you call from your US phone. Its a long distance call (even though you are in CR too) and she's not going to pick up because its going to use up her talk minutes. No one under 40 answers the phone anymore. Everyone texts.
Oh, and you can send pictures

, video

, and documents (like PDFs) over WhatsApp (I even get invoices from companies that way in CR). And make video calls (the better / faster the WiFi the better the video).
And did I mention its end-to-end encrypted?
But you can't sit in the SL and scroll through chicas (that aren't in your contacts) and 'order in' chicas like a digital monger.
What you can do is look at your WhatsApp contacts (integrated from your contacts in your phone that are on WhatsApp) and think "I wonder if Luna is available?" and then send her a WhatsApp.