Flixster Inc, the privately held San Francisco company, made the announcement this morning. In case you don’t know, Flixster is a movie rating website, and Rotten Tomatoes is also a movie ratings site that focuses less on social networking and more on critical response and aggregation. Both sites say they’ll continue to operate as separate properties, though probably with lots of synergy like integrated data. Combined, Flixster and Rotten Tomatoes claim they’ll reach an estimated 30 million monthly visitors worldwide across several Web platforms — their sites, other social-networking sites and applications for mobile devices. They say they’ll have a database of more than 250,000 movies, 2.3 billion user reviews, half a million critic reviews and more than 20,000 trailers and videos. Before the acquisition, Flixster and Rotten Tomatoes had partnered in several areas, including a recent deal that syndicates critic reviews from Rotten Tomatoes to Flixster’s online movie community, both on the Web and via Flixster’s mobile apps.

“Rotten Tomatoes has built a fantastically well-known brand that moviegoers trust when making their decisions. Combined with Flixster’s social networking and word-of-mouth, we’re creating the leading movie destination on the Internet, ” said Flixster President/COO Steve Polsky. News reports say the deal comes as IGN is working to refocus its efforts to build out its portfolio of video game-related and men’s-lifestyle offerings.

“Joining Rotten Tomatoes with Flixster creates a company that can dominate the online movie category,” said Roy Bahat, president of IGN Entertainment, who will join Flixster’s board of directors as an observer since IGN also will get a minority equity stake in Flixster as part of the acquisition. “This also enables IGN to focus on serving the male 18-to-34 audience – especially videogamers – and the advertisers looking to reach them.”

UPDATE: *This follows an AllThingsD report centered on Rotten Tomatoes merging with Flixster in exchange for a stake in the combined independent company by News Corp. (There was also a previous thought that the bigger News Corp company MySpace might buy Flixster which would then combine with Rotten Tomatoes. There’s no sign of MySpace in this deal, however, although News Corp will take a minority stake in the buy.)*

Several web experts are calling this an acquisition that makes sense. Mashable’s Christine Warren writes: “Far too often we see companies acquire other companies that seemingly bring very little value to the core mission. In this case, Flixster and Rotten Tomatoes actually seem like a match made in movie lover heaven. Rotten Tomatoes has a strong existing brand and is considered a trusted go-to source for movie fans looking to find out what the general consensus is surrounding a film. Flixster already has a really strong hold in the web and mobile app space — with applications available on the iPhone, BlackBerry, Palm Pre and Android platforms. Rather than trying to compete against each other, it makes much more sense for the two companies to come together.”