Inside Sonix, click EXPORT and select SubRip subtitle file (*.srt)
Then once inside Facebook follow these steps:
Click Photo/Video at the top of your timeline or News Feed.
Click Upload Photos/Video.
Choose a video from your computer, then click Post.
We'll notify you when your video is ready to view. Click the notification or the gray date and time at the top of the post on your News Feed or timeline.
Hover over the video, click Options at the bottom and select Edit this video.
Click Choose File below Upload SRT files and select the .srt file you exported from Sonix.
To add captions in other languages, repeat steps 4–7 for each additional language you want to add.
To remove captions from a video you've uploaded:
Click your video to expand it
Click Options at the bottom and select Edit This Video from the menu
Click the X next to the file you want to remove