Pros & Cons

Pros

Your advocacy messages will be broadcast to exponentially larger audiences.
You will reach, recruit, and mobilize new supporters that would not otherwise have found their way to your website.
Your share of influence at social networking and social media sites will be greater, as an active participant, rather than as a bystander.
Links promoted to the front page of social media sites frequently lead to coverage (and further links) by bloggers. Even when the buzz and surge in traffic to your website subsides, the links pointing to your website will remain, which will help increase your ranking in search engine results (at least a little).
Supporters at social networking sites will spread your message to their friends. This "word-of-mouth" promotion is free and often effective, since friends are more likely to pay attention to their friends.
Your webpage at social networking sites will be a good source of ideas and feedback from supporters, would-be supporters, and detractors.
Even accounting for staff time to manage Internet Outreach, cost per impression for your advocacy content can be very low.

It's easy to waste time: If you don't manage your staff and volunteers carefully, they can easily confuse effort with progress. For instance, it doesn't really matter that you have amassed 83,000 "friends" at MySpace if they don't help you accomplish a larger objective.
You may be embarrassed by some of the well-intentioned user-generated content created to support you. ("With friends like these, who needs enemies?") However, you cannot be successful at Internet Outreach unless you're willing to at least "share" control.
If your most appealing content is freely available at social networking sites, there is less reason for internet surfers to sign-up as members at your own website.
The effectiveness of Internet Outreach defies precise measurement.
Someone on your team may get so excited about early and amazing promotional success that they will push too far and get banned as spammers. That may cause your whole team to become unnecessarily cautious in the future.
You will face copyright and fair use issues that you've never had to deal with before.