Social networks are growing bigger by the day.
Unfortunately, the social network is outside of your control.
I believe that social networking is the heart of the web. I love the fact that people are actively linking to eachother on twitter and facebook.
However, I think that one's web site or blog should be the center of their social networking effort.
Practicing what I preach: My twitter account has some 9,300 followers (mostly robots). I've put hit counters on tweets. In most cases I only get about 40 hits (from mostly robots).
My personal networking site is Community Color. This site has over 20,000 active links and has listed over 20,000 community events in the calendar. (NOTE, I've removed 7,700 dead links from the directory). I've placed hit counters on entries in the directories and generally get several hundred hits per listing.
More importantly, direct links from home pages build Google Page Rank. Google uses PR to sort search requests.
Twitter blocks Google from crawling its pages. The links are marked rel="nofollow". Tweets do not help web sites build pr. A tweet or follow might send a few tweets in the direction of a site you support ... but it doesn't really help others build sustainable traffic.
If there is a cause you support, you do better to write a blog post about the cause; then tweet about the blog post.
BTW community color makes directories for towns in the mountain west (Utah, Colorado, etc). For example, gjct.com is for Grand Junction. I just started making a directory for Arizona. The Stats page shows the number of links broken down by directory. Provo has some 1600 links, Salt Lake City has some 5700 links.
In conclusion, your personal web site or blog should be the center of your social networking effort. Linking to local organizations can help sustain a vibrant local community in the internet age. I am proud of the fact that my local link list is still magnitudes larger than my twitter follow list.