The solution is simple - say the tipping point is 20 friends must do X for you to do it. So walk the whole network and find everyone with more than 20 friends. Remove those with the most friends (say the those > 99th percentile).
Now walk the network again, and find those with more than 20 friends and again 99th percentile goes. Eventually you remove the 20th friend from everyone and those left have 19 or fewer friends.
Now tell all these seed group to do behaviour X.
Now put back the very last set you removed. And they are guaranteed to have at least 20 friends, and all those friends will be doing behaviour X. Now put back the penultimate group, and because the most recent arrivals are now also doing behaviour X ....
Problems:
1. For any social network at a given point there is just one seed group. Right now (or 6 days ago) this group is being identified. And sold.
2. the tipping point theory as a whole is a bit dodgy (see below).
3. Feasibility - they only mentioned orders of magnitude smaller subsets. Lets be kind and suggest that its 3 orders of magnitude. for LinkedIn that leaves a seed group of 100,000's. Not the size you can just invite into a focus group.
Overall, really a cool hack, and I swear its worth ponying up on and selling to excited digital agencies
(#) Good article on Debunking of tipping point theory - http://www.fastcompany.com/641124/tipping-point-toast
Edit: Wanted to rewrite my below comment that got a bit confused