Create a drawable with name com_facebook_button_icon.xml add anything inside it FB login button will override as it uses as drawableleft.
For example:
<vector
android:height="25dp"
android:viewportHeight="1365.3333"
android:viewportWidth="1365.3333"
android:width="25dp"
xmlns:android="http://schemas.android.com/apk/res/android"
android:tintMode="multiply"
android:tint="@color/com_facebook_button_text_color"
>
<path
android:fillAlpha="1"
android:fillColor="#FFFFFF"
android:fillType="nonZero"
android:pathData="m1365.33,682.67c0,-377.03 -305.64,-682.67 -682.67,-682.67C305.64,-0 0,
305.64 0,682.67 0,1023.41 249.64,1305.83 576,1357.04L576,880L402.67,880l0,
-197.33l173.33,-0l0,-150.4c0,-171.09 101.92,-265.6 257.85,-265.6 74.69,-0 152.81,
13.33 152.81,13.33L986.67,448L900.58,448C815.78,448 789.33,500.62 789.33,
554.61L789.33,682.67L978.67,682.67L948.4,880L789.33,880L789.33,1357.04C1115.69,
1305.83 1365.33,1023.41 1365.33,682.67"
android:strokeColor="#00000000"
/>