You can share only share the image or images on Facebook, there is no text option, because due to a Facebook policy the predefined sharing text is not allowed, and will be ignored by the Facebook app anyway.

Sharing approach is the no different from sharing to another social media with Ultimate Mobile.

using SA.Foundation.Utility;
using SA.CrossPlatform.Social;

var client = UM_SocialService.SharingClient;
var builder = new UM_ShareDialogBuilder();

//Juts generating smaple red tuxture with 32x32 resolution
Texture2D sampleRedTexture = SA_IconManager.GetIcon(Color.red, 32, 32);

client.ShareToFacebook(builder, (result) => {
    if(result.IsSucceeded) {
        Debug.Log("Sharing started ");
    } else {
        Debug.Log("Failed to share: " + result.Error.FullMessage);

Please note, that you can use native sharing only if the application you want to share with is installed on a user device. There is no cross-platform API for detecting if the application is installed on the device since approaches of iOS and Android are very different. But both platforms provide the way to detect if the application is installed, check out the per-platform guides: