I'd be very worried about sending my sensitive data (passwords, personal info, etc) through these kind of sites. Just my 2 cents.
There's other sites that still accept Americans: The Merge network (Carbon + other skins), Bodog, Cake network, WPEX (65% rakeback for all, no referrals/codes needed)...
Traffic might not be all that high, but as Americans migrate, the sites will grow. Recall that before Party pulled out of the US market (way back when), Fulltilt was actually quite a small site. Then the Americans migrated from Party to Fulltilt/Stars, making Fulltilt a lot larger than it was.