Not to forget that for many, many users in emerging markets, the mobile carriers offer free data for WhatsApp. Traveling through Colombia, and I can use WhatsApp status for free over here. SnapChat kills my data.
I was about to write the same thing. Here in Sweden you often see adverts for telcos where they offer free data for Facebook usage and stuff like that. We're not a developing country and you'll get a fair bit of data at a reasonable cost here, so it's perhaps not as obvious how this skews things in favour of the big players, but it's there alright, and it's sad.