Posterous Autoposts to Facebook in a number of ways. There are several options you can set that will change the way Posterous autoposts to Facebook. By default, if you email (or compose in the web editor) a text-only post to Posterous, it’ll be Autoposted as a stream update with a link to the full text of the post on your Posterous page.

If you include photos in your post, by default, we’ll Autopost to either the Posterous Photos album , but won’t include a stream update in addition to uploading the photos included in your post. This is because uploading photos to an album automatically generates a Facebook stream update and many users do not want multiple stream updates generated for a single post.
However, you can change all of these settings by clicking the pencil (edit) icon next to your Facebook Autopost in the Autopost section of your Posterous Space Settings:

In these settings, you can adjust the following:

Autopost photos to my albums This determines whether Posterous uploads photos to a Facebook album

Don’t post stream story This determines whether Facebook will post a stream story when photos are uploaded (uploading photos automatically generates one stream update)

Minimum gallery size This determines the number of photos required before Posterous creates a new album (with the title of the post as the title) instead of uploading photos to your Posterous Photos album

Autopost videos to my albums This determines whether Posterous uploads videos to your Facebook account