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

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)