http://fossil.energy.gov/programs/gasregulation/publications/3rd10ng.pdf
These are 2010 totals:
Imports:
Total: 2880.5 bcf
2518 bcf from Canada via pipeline
25.2 bcf from Mexico via pipeline
Remaining 336.6 bcf via ship from various countries.
Exports:
Total: 773 bcf
510 bcf to Canada via pipeline
240 bcf to Mexico via pipeline
23 bcf to various countries via ship, the largest of which is Japan.
Now, they don't break down those imports and exports by the type of well. Most of these wells dump the gas into pipelines, which are the same pipelines that other types of wells are dumping into, so it makes it tough to break down that way.
Looking to turn this into a %, I found a source that said in 2007, the total gas production in the U.S. was 19.3 Tcf. While there's error here because the exports are 2010 numbers and the production is 2007 numbers, as a country we export about 4% of the gas we produce, and imports are about 15% if compared to the amount we produce domestically. So still a trade differential, but not nearly as bad as oil.
This is largely due to Canada, by far our biggest supplier, and also our biggest customer. I'm pretty sure our interior areas don't export much, and don't import much. It's the border regions where it all takes place, because very little is shipped overseas. Again, a different world than oil, I think our relations with Canada are rather stable.
Non-conventional gas plays, which includes but is by no means limited to Marcellus, contributed 42% of our nation's gas in 2007, and that's projected to increase to 64% in 2020. I have no idea if the % of the exports fits this or not, but I suspect it'd be strongly influenced by geography. Again, the sources closest to the Canadian border are going to contribute a higher % to exports than sources in the interior.