system("finger$_  $_");

Let's say one of your hosts in hosts.txt is named "foo", then your command line would expand to

    fingerfoo foo
[download]

Probably not what was intended...

Update: perhaps you meant something like "ssh host last -n1"¹ ? (see "man last")  With finger, you'd need to specify some user ("finger user@host"), so this probably isn't what you want — and depending on your site configuration, remote fingering (i.e. @host) may not work at all.

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¹ after seeing Utilitarian's reply, I realize that "the last login and time for every system" can be read in several ways. The way I understood it was that you wanted to know when anyone last logged in on those hosts, not from those hosts to your local machine...