At the moment rich format can be only used when creating posts for Facebook, Instagram, Wordpress and other blogging sites (such as Tumblr).
If for some reason the rich format doesn’t appear in your Compose box for any of these social sites, you can turn rich format manually on by going to your Settings > Preferences > Publishing Preferences > Rich text editor.
You can here set your rich format to come up in your Compose box to:
Automatic - rich format should appear for only the above sites,
Always on - select this if you want the rich format to appear in your Compose box all the time, or if the rich format didn’t appear for any of the sites mentioned above.
Always off - rich format should never appear at all.
With the rich format option "Always on", in your Compose box rich formatting will always display even for the sites not supporting rich formatting.