Researcher iStrategyLabs just released new numbers on Facebook user demographics, and one figure stands out: the number of members who are 55 and older grew 25% in the last month alone. The segment more than tripled the overall rate of growth for U.S. users on the site in the month ending August 4, which was 8%.

Here’s the chart:

Much has been made of the aging of Facebook, but the sustained rate at which baby boomers and seniors are joining the site is surprising. It seems that we’re past the point where you could call this a fad — older folks are continuing to join the site because they’re seeing their friends and family members living their lives on the site and don’t want to be left out.

What does this mean for Facebook? It’s a valuable opportunity for advertisers on the site, who would like to reach older Americans with more buying power. But the company will have to also think about how the site should evolve to accommodate the increasing diversity of its members. For example, baby boomers appear more interested in intimate communication with their family, and some of them feel like they can’t get that on Facebook.

iStrategyLabs gets its numbers from Facebook’s Social Ads system, so they’re reliable. But it’s important to note that this information is not obtained through surveys — it’s pulled from user profiles. That means that whatever personal details users leave out of their profiles — such as school affiliations, home towns, and even ages — won’t show up here.

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