Celebs do it all the time on public TV. (promote products they probably don't use let alone like).
Uh huh, and they get paid to do just that. If I were asked to do this, I'd ask if I have permission to use company resources to do so - including time, bandwidth and online reputation. If the answer's no, I'd ask for compensation for using my personal resources to do this work for them. I'd also consider asking to have my job description altered.
Tell your friends you're astroturfing outside of the social networking sites and move on with your life.
That's not necessary if his glowing endorsement of "Acme Widgets' stunning new app ParaWidget-X" comes from employee@acme.com
My advice to the OP? Face facts. Your employers are asking you to do unpaid work for them. Change the deal to a paid one by conducting this advertising campaign on their time. Protect your personal online reputation by creating a twitter/facebook/whatever account under your employee email address. Then spam away, my friend.