How do I pass a URL with multiple parameters into a URL?

You are missing the ? in the second URL (Also, it should be URL-encoded to be %3F).

Also, I believe that the remaining & need to be URL, not HTML-encoded. Change &second=12&third=5 to %26second=12%26third=5 and everything should just work.

This:

&u=http://www.foobar.com/first=12&sec=25&position=2

should be:

&u=http://www.foobar.com/%3Ffirst=12%26sec=25%26position=2

In your example parts of your passed-in URL are not URL encoded (for example the colon should be %3A, the forward slashes should be %2F). It looks like you have encoded the parameters to your parameter URL, but not the parameter URL itself. Try encoding it as well. You can use encodeURIComponent.

Rather than html encoding your URL parameter, you need to URL encode it:

http://www.facebook.com/sharer.php?&t=FOOBAR&u=http%3A%2F%2Fwww.foobar.com%2F%3Ffirst%3D12%26sec%3D25%26position%3D

You can do this easily in most languages - in javascript:

var encodedParam = encodeURIComponent('www.foobar.com/?first=1&second=12&third=5');
// encodedParam = 'http%3A%2F%2Fwww.foobar.com%2F%3Ffirst%3D12%26sec%3D25%26position%3D'

(there are equivalent methods in other languages too)