Well, the primary motivation for organizing would be the benefits I listed - the ability to get fringe benefits cheaply, and the ability to have clout on workplace policies.  The Freelancers Union has had good success organizing freelancers in New York, and many of those freelancers are in competition with each other, so there's reason to believe that could continue.

I agree that strikes might not be effective online, although I might be proven wrong.  If eBay sellers chose not to sell any products during a key week right before Christmas, for example, that could feasibly have a significant impact on revenues.  Some non-unionized sellers might fill the void, but it's not like any old yahoo can become a seller with a good reputation overnight; if a significant-enough share of the user base was unionized, the tactic could still be effective.  But on the whole you're right; online workers would probably need to find another way to gain leverage with their workplace.