[gowld]

Peak for you was likely when you were a teen.

But overall? In 2001 computers weren't ubiquitous nor portable. Middle-class kids were on IM on their desktops and maybe laptops for max a few hours per day.

Now kids have phones for IM all day and night.

[cortesoft]

The grandparent comment was saying that 2006-2008 was peak instant messenger, and people using messengers on phones were certainly not common during those years.