WhatsApp and SMS (text messaging, including Apple's iMessage) are now part of our family of share buttons! These buttons are "smart" ie. they are designed to only show when a visitor is on a supported mobile devices since these are mobile-specific services. This optimization happens automatically for you, so you don't have to worry about it!
Update: As of February 2018, WhatsApp will now be shown on Desktop computers as well! So if you have WhatsApp selected as one of your Share Button services, visitors will now see this button loading on both Desktop and Mobile devices!