Here's some basic code for an Android app which will post the URL of your app to Twitter. Stick it in a button or menu item for easy sharing.

String twitterUri = "http://m.twitter.com/?status=";
String marketUri = Uri.encode("http://example.com/?q=app&title=test");
Intent shareOnTwitterIntent = new Intent(Intent.ACTION_VIEW, Uri.parse(twitterUri + marketUri));
startActivity(shareOnTwitterIntent);

Some important things to note.

1. This is set to post to the mobile version of Twitter.  Your user is on a phone - don't direct them to a site that won't work on their device.
2. The second string is URI encoded.
3. Consider if you want to post a "market://" link.  I would advise against it.  Twitter won't render it as a link and, even if it did, 90% of users won't be able to click on it.  Make it a link that will direct desktop users to your website, mobile users to a mobile friendly site and Android users direct to the market.

Facebook also has an API for this sort of sharing.

http://m.facebook.com/sharer.php?u=example.com&t=test

Again, it points to the mobile site and needs to be URL encoded.

Happy sharing!