While everyone constantly seems to be talking about Twitter lately, the social discovery network StumbleUpon just signed up its 7 millionth user, making it nearly twice the size of the microblogging service. Although the services are pretty different -- Twitter's all about the chatter, while StumbleUpon is something a little closer to what Marshall Kirkpatrick calls "brain therapy" for the Web 2.0 world -- StumbleUpon has a well-established record of delivering traffic to Web sites and a clear pathway to profitability with paid stumbles.

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