Twitter‘s management seems to have mixed feelings about advertising. But here’s anecdotal evidence that those text ads it has been running lately are worth something.

StockTwits, a site for sharing stock-related tweets, saw a sharp uptick in users last month, around the same time that Twitter began running ads for it.

In March, StockTwits had nearly 50,000 users but grew to more than 210,000 in April, according to Compete. Around the same time that Twitter saw a sharp traffic spike. To be sure, StockTwits has been getting coverage in the financial press, and has spread through word-of-mouth on Twitter and the web, as cofounder Howard Lindzon explained to me today.

So, there are various reasons why StockTwits might be growing fast all of a sudden. But the combination of Twitter itself growing and the ads certainly correlate with StockTwits’ growth. (Note: The screenshot is of Twitpic, another service that uses Twitter, as I don’t see StockTwits showing up for me at the moment.)

Remember, Twitter isn’t charging for these text ads at this point — if you can even call them ads. In fact, as StockTwits cofounder Soren Macbeth told me in last month:

I saw that [text ad] box there with other Twitter apps and asked for StockTwits to be included. Next thing I know there is it was. That’s unfortunately all the information I have about. I can say that it is quite effective in driving traffic though.

At a conference last week, Twitter co-founder Biz Stone said that the company isn’t especially interested in traditional advertising versus other ways of making money (something it hasn’t done at all yet). But then he published a company blog post clarifying that it’s not opposed to this form of revenue. Given how these text ads seem to be working for StockTwits and presumably the other featured companies, it might be time to start charging for them.