I don’t know about other sites, but for Fluther it has happened gradually. In its early months, many of the members screen names had some variation of the name “Finkel” in them. Gradually over time, people began to read about the site and join. A big boost came when it was featured as an iPhone app and we hav had numerous influxes from othe Q&A sites as people became disenchanted with them. There has also been publicity through magazine and newspaper articles, interviews of the founders and the SxSW conference in Austin.