The “slightly smiling face” is currently only present in Twemoji as an .ai asset. I’ve asked in #67 and #59 about how to move forward to include svg and png versions.
It is already present in Twemoji as mentioned above (although as an .ai file only so far); according to [300+] Emoji Wallpapers | Wallpapers.com it is present in Emoji One; and according to this table it is present in the Windows set. I assume the emoji sets are you referring to are Windows, Android, Apple and Twemoji, right? If so, only Android and Apple are is (apparently) behind. Note that it was introduced only in Unicode 7.0.