Twitter isn't just dealing with its well-documented tech problems (both with the service and its personnel). It's raising a Series C round. So what's the valuation?

Last summer, Twitter raised about $5 million at a $20 million valuation. This time, we hear its looking for about $15 million at about a $60 million valuation. We hear the company is talking to the "usual suspects" -- we assume this means existing investors like Union Square Ventures and Charles River Ventures, as well as players like Spark Capital -- and hopes to have a deal done soon.

Some sources are also passing along stories of Twitter attempting to seek a much higher valuation -- as much as $150 million -- but finding no takers. When we've bounced those numbers of people with first-hand knowledge of the company, however, they choke and sputter, so we wouldn't put much stock in them.

The bigger question: How do you put a value on Twitter, anyway? The company has only just started seeing a trickle of revenue, via advertising on its Japan version. But beyond that there's no money coming in, and it's not clear what the model will be.

While Twitter itself has great buzz, we hear the majority of the site's traffic comes from outside the site, via other apps like Twhirl, mobile access, etc. So traditional online advertising--a difficult prospect to begin with for a communications service (see the struggles of various IM, email platforms) may be even harder.

That said, based on Twitter's growth and brand dominance, $75 million post-money seems plausible. There must be a pony in there somewhere.

See Also: Twitter Turmoil Not Over