+ 1 on Randar's comment ...
1) since when you are starting out, you don't know precisely what your best sources for great candidates will be, the multi-channel approach is where you want to start
2) measuring which channels are most effective for while reqs is critical in optimizing your ongoing efforts
3) networking is always be a channel, unless it's not working at all, but if it's not working over time, I'd ask why not. For instance, when you start to hire, say, customer support people, it's possible those profiles aren't in your network, but over time, if your network to these profiles isn't growing, then you're probably missing something critical
4) I get frustrated with inbound applicant volume and low success with job boards as much as the next person, but I also think it's a mistake to completely discount these channels (see Randar's results). One thing to consider on the inbound front is employer branding - at Inflection, we have strong employer branding, which has translated into pretty solid inbound pipeline.
Bottom line: try everything, be creative, measure, optimize