wp8 api question

[zeronoise]

Just got my first windows phone (from iPhone) and love it. My main question is whether the messaging app API is available to devs...i know the phone app is so devs can integrate directly into the phone but is the messaging app? I'm curious because I saw that Facebook is integrated (this is microsofts doing) but would a third party dev be able to do the same? For example whatsapp. Thanks