Here is a nifty tool: Is It Worth It? An ROI Calculator for Social Network Campaigns:

You can use this tool to calculate an estimate of cost and return on investment for the recruitment and fundraising efforts of your staff in social networking sites like Facebook or MySpace. It works sort of like an online mortgage calculator. Just enter the starting assumptions in the yellow boxes below and the tool calculates results automatically.

This web-based spreadsheet (you edit the variable values right on the page and then click the ‘Update’ button to recalculate) might help you to understand the cost of investing time and effort into social networking compared to what you might realize from it. This tool is designed specifically for fundraising but you could probably use it for membership recruitment as well.