For quite a while now, different public personalities(or at least their PR people) have been using LinkedIn’s Questions & Answer’s feature as a communication tool to reach the business world-wide professionals community. The recipe is simple and very effective: post a question on the Q&A section featured under the profile of public person and you’ll surely get ‘000s reactions, and along with them, exposure to the information posted under your profile.
The goals behind such a choice of communication are probably quite diverse as well. While for Barack Obama its clearly a matter of campaigning, when it comes to the question (you must be logged in) featured under Bill Gates’ profile, my best guess would go towards CSR moves or such. In any case, this communication method is becoming more and more popular and it’s easy to see why: easy (as in very fast/cheap etc) tapping into the views and opinions of millions of highly educated people is a great resource for whatever your information needs would be.
Here’s Bill Gates’ headline – he only has 3 connections so far, so I guess his very selective with his invite requests

