Did LinkedIn more than double its U.S. visitors in October? A casual glance at the latest comScore data makes it look that way, with LinkedIn shooting up to 20 million unique visitors in October, 2009, from 9 million in September, 2009. The new estimate puts LinkedIn ahead of Twitter, which saw a decline of U.S visitors last month to 19.2 million.

Of course, LinkedIn didn’t really all of a sudden have a growth spurt in October. Rather, comScore was previously under-counting its reach. Now it does a better job of measuring Web usage at work, which apparently is where about half of LinkedIn’s users check in. It makes sense, since LinkedIn is a professional network and you are more likely to be looking for contacts at other companies during work hours.

While the fish-shaped chart above suggests that LinkedIn is now about as popular in the U.S as Twitter (or at least Twitter.com), the two are complementary. On Tuesday, LinkedIn gave users the ability to link to their Twitter accounts so that their Twitter stream shows up as status updates in LinkedIn as well. Since Tuesday, LinkedIn has witnessed a 25 percent increase in status updates. Long live the stream.