Different strategies to build relationships in the social media world. I thought the idea of becoming an informal community manager for someone else's community was very interesting. 

True; this is a great idea:

3. Become their informal (or formal) “Community Manager”

A very powerful secret to adding value and to getting to know an influencer is to fill a vacuum in one of their communities.

If they have a following on LinkedIn, Facebook, or in a forum (it can be free or part of a paid program), proactively take on the role of an informal “community manager.”

Help newbies get oriented. Create mini-tutorial posts or videos. Help answer questions that might be slipping between the cracks.

As a LinkedIn Group owner, I can tell you that the people who help me with these things in my community are worth their weight in gold. Anytime they contact me, they immediately have my attention. As an added bonus, you’re also building authority and good will with the other members of that community.