On the day Facebook finally filed for its IPO, an analyst signalled that Google+ is slowly creeping up on the number one social network.

Google+ watcher Paul Allen estimated in a blog post that the social network has reached 100m users, an increase of 10m since Google last announced membership figures just two weeks ago.

This means Google+ has been adding users at a rate of 750,000 per day.

Allen, who has been accurate with his previous estimates, works out the statistics by running analysis on the number of users with hundreds of different surnames.

He believes it will reach at least 400m users by the end of 2012.

“Google has many dials and levers to use this year to increase signups and usage, including its Android 4.0 rollout.”

In a previous post Allen said that increased usage of Chrome and YouTube will also contribute to raising the profile and user figures for Google+.

Google’s decision to open up the network to teenagers and the launch of Search Plus Your World, the much-publicised integration of Google+ into search results,  will also help boost its figures.

It’s getting to the stage where very few people will be unaware of Google+, but convincing them to sign up and use it on a regular basis is a different challenge.

Anecdotally, of the people I know who have signed up for Google+ most did it out of curiosity and have since logged on rarely, if ever. 

So while the number of accounts is impressive and growing, statistics around engagement and time spent on the site are more important when analysing the relative strength of Google+ compared to Facebook – which, as we should all know by now, has more than 800m active users.