The sun is shining bright for Tumblr today. The microblogging platform raised $85 million and was heralded as "bigger than Wikipedia." But Tumblr's explosive growth has been driven by huge quantities of porn and spam, and that seems to make the startup uncomfortable.

Despite $125 million in funding and a reported $800 million+ valuation, Tumblr is still bereft of advertising. As it goes looking for revenue, the company frets about its trove of porn; Tumblr unveiled a hand curated directory of erotica last year, only to pull it. Despite the popularity of its "porn" tag, it's not listed on the "explore" page with other popular categories.

There's no question Tumblr is rich with dirty pictures. Just look at Fleshbot's Tumblr tag, this roundup of search terms, or this 2009 survey of Tumblr subdomains. When we plugged "Tumblr" into Google's advertiser keyword tool, two of the first five results that came up, sorted by relevance, were For Her Eyes Only and The Girl Next Door, both NSFW Tumblrs, while a list of most-searched-for terms involving the word "tumblr" included, in the top 30, "sex tumblr," "tumblr cock,""tumblr cum," and "tumblr breasts," all of which produce very relevant results, e.g. top hits For the love of cock, Cum Whore, and the accurately named Face And Breasts.

None of this is to say we disapprove morally. Tumblr's porn, like the rest of its blogs, tends toward the well designed and smartly curated, particularly in comparison to the flashing, browser exploding mess you'll find elsewhere on the web. We'd totally click "FOLLOW" on some of this stuff!

But the obvious prevalence of Tumblr smut, combined with the company's public ambivalence about it, means Tumblr's traffic is not as sustainable as, say, Wikipedia's. It's not hard to imagine Tumblr cracking down on adult content to appease advertisers; it would hardly be the startup's first controversial or heavy handed content intervention. If Tumblr were to begin evicting porn, Richard Branson, Greylock Partners and the other VCs behind Tumblr's latest funding round would find the company's numbers had become suddenly less impressive.

Porn isn't the only issue Tumblr investors should be asking about. Spam has become a growing problem on the blog network; a chain letter scam this summer netted 130,000 victims, and users are not shy about complaining. Meanwhile, as it grows and its bank account swells, Tumblr becomes a more obvious target for copyright complaints. DMCA provisions of copyright law might limit the company's liability, but there's a fair amount of Tumblr sharing that simply could not go on if copyright rules were strictly interpreted and enforced.

It's nice to imagine that Tumblr's investors have done their due diligence carefully weighed all these issues and decided the company's future is safe. But venture capital doesn't work that deliberately even in the slowest of markets, to say nothing of the present bubble. Whether Tumblr carefully nurtures its network into something lasting or blows its wad prematurely chasing an unrealistic fantasy is pretty much up to Tumblr at this point.