Tap to tweet/tap to post is at the bottom of the notification center
In the Notifications section of the Settings app, you can tap the Edit button in the upper right to make handles appear (three thick horizontal lines) on each of the items  in Notification Center.

 

You can drag the items up or down by these handles to re-order them, so just drag the Share Widget item to the top, if that's where you want it, then tap done.