Simple email is easy for one or two people to handle, but from what I've seen Twitter/Facebook interaction (which also doubles as an advertising route) requires dedicated staff, and some companies even outsource the handling of PR through those just because it is expensive and difficult.
For everything small, a help desk tracker is great if your service is technical and your users are technical, it keeps tracking of issues much easier.
However, if you go the route of email, you would still need a tracker internally, and some companies have even plugged their email system into their internal tracker to make this easier.
olark is great because its much lower barrier of effort than email, so your customers often come with questions they might not have bothered with via email. You can get some great insights into how people use your site, and you'll learn about bugs much faster.