I haven't read other responses...but bare in mind that the mouth of an infant is used for sensory so it could be a combination of things. Often times because it will still be some time before their vision is fully developed, putting things in their mouth (like hands) is their only way of exploring until they're able to physically grasp other objects to investigate. Plus, they may like the sensation of wiggly fingers in their mouth and this fascinates them.
I still wouldn't rule out teething though. My son started around 11-12 weeks drooling like a Saint Bernard, but the first one didn't actually come through until about 5.5 mos.