“We’re not a social network, we’re an information network.” That’s what Twitter’s vice president of engineering, Michael Abbott, told the crowd at Mobilize 2011 this week. That isn’t new branding, Twitter has been denying it’s a social network since at least a year ago. It all dates back to November 2009, when Twitter changed the question it asked users for status updates from “What are you doing?” to “What’s happening?”.

The trouble is, Twitter’s main rivals Facebook and Google+ are not sticking to their knitting like Twitter is. Last week, Facebook widely expanded the range of information it tracks: read, listen, watch, 5 types of “life events” and more. Meanwhile Google+ has become known as a place for people to discuss common interests. This is all bad news for Twitter.

Twitter used to be seen as competing with Facebook as a social service. But that was a fight Twitter was never going to win. So, wisely, it shifted the focus to being an information network. That strategy has worked up till now, because Twitter has built up a huge store of what the Web 2.0 Map calls Interest Data – defined as “declarations of what people are interested in.”

The problem for Twitter is that both Facebook and Google+ have begun to target the information network business too.

Just last week, Facebook opened up its platform to Read, Watch, Listen data. If users of Facebook allow apps such as Spotify (music), Washington Post (news) and Netflix (video) to automatically share data to their Facebook News Feeds, then suddenly there is a lot more Interest Data flowing through Facebook. Also expanded types of status updates (so-called “life events”) will add to Facebook’s Interest Data store.

Already I’ve found myself using Facebook more to say what I’m reading, listening to or watching. Up until very recently, whenever I liked a song enough to publicize it, I usually did so on Twitter. But this morning I listened to the new Wilco album for the first time and used Facebook to tell people about that. That was both a social and interest activity.

The upshot is that Twitter needs to watch out for Facebook, as Big Blue F wants to dominate both social and interest data.

Meanwhile, Google+ is increasingly being used as a way to interact with your interest network. Despite the anti-Facebook declarations of Google+’s more fervent supporters, in fact Google+ is less of a social network than pundits initially thought. If your family and many of your close friends aren’t on Google+, then is it really as good a social network as Facebook? It’s not about the feature sets, it’s about the people. Facebook wins right now on the people front.

Robert Scoble left the following comment on my Facebook wall earlier this week: “I am seeing the two diverge in the future. G+ will be more for common interests. Facebook will be for more your social graph interactions.”

While that does appear to be the current trend, Facebook clearly wants it all. The big question will be how much of a dent Facebook makes on Interest Data, versus Twitter and Google+.

So Twitter has its work cut out, despite the impressive growth it continues to experience (it claims to have 100 million active users now). Twitter has already lost a portion of its early adopter audience to Google+ and now has Facebook breathing down its neck.

What do you think, can Twitter continue to grow with such intense competition from the big guns?