[djulius]

It was ubiquitous, everybody had a minitel at home since it was provided for free by France Télécom.

The white/yellow page service (3611) was free for the first three minutes and was widely used. My parents never had the (huge) yearly book version at home.

As mentioned in other comments it was also widely used to consult Baccalauréat (national exam like SAT) results.

[bigbugbag]

At peak in 2003 there were 9.1 millions minitels for 55 millions people so not ubiquitous but common.

3611 used to offer the first 3 minutes free for a few years then removed this.

[vbernat]

3611 was free until 2007, when it became mostly irrelevant. 9.1 millions of minitels is about half the population (in number of households). I don't think that 2003 was a peak since Internet was already there for quite some time (people could get broadband at this time).

[Renaud]

People had a minitel per family, not per person, so nearly 10 million minitels make for a pretty good penetration rate.