LinkedIn -- it's now considered "essential" for professional networking, especially if you're looking for a job. Chances are that your connections won't know of a job, but their connections probably will.
...But what happens with the LinkedIn mobile app when you try to click on "add all connections"? I used to have an Android, which synced all of my Google contacts. This included anyone who had emailed me and anyone whom I had emailed for various groups (from my department to the Hopkins Imaging Conference to the Association of Women in Science). Some of these emails weren't even on behalf of my own name, but were sent from the email as the head of a group (e.g., "firstname.lastname@example.org"). Needless to say, when my phone found all of these people's email addresses on their LinkedIn accounts, and then my finger accidentally brushed against "Add All Connections," and then there was no "Confirm" button.... a LOT of invitations got sent out.
Now, many folks accept everyone who's added them. This is a standard and encouraged practice for building a network. However, I'm a little uncomfy with being linked to just anyone, so I'll usually only accept or add people whom I've met in person. Or if someone sends me a personal message with their reason for wanting to connect, I'll consider accepting. So how to deal with this influx of random connections?
Well, you can go to all of the people who have accepted this blanket invitation (and there will be folks that accept your invite within the first few minutes of your sending it) by clicking on "Connections" --> "Recent" (described here: http://help.linkedin.com/app/answers/detail/a_id/49/~/removing-a-connection). However, this eliminates the ability for either of you to message each other unless you initiate a connection again.
Apparently, you can also go to your Inbox --> Sent, click on individual messages to "Join my network on LinkedIn," then "Withdraw" invitations that have not been accepted. To withdraw all pending invitations, you need to email LinkedIn's customer service through the "Contact Us" page on the forum (described on http://community.linkedin.com/questions/218617/how-do-i-undo-a-invite-all-to-contacts.html).
Very clunky. LinkedIn, you need an "undo" button. Or, at the very least, a "confirm add all contacts?" button.