If the scripts are loaded in the normal, synchronous way, then just make sure that your <script> include appears after the library scripts in the document’s <head>. If, on the other hand, those scripts are loading objects asynchronously (as seems to be the case), then create something like this:
function whenAvailable(name, callback) {
var interval = 10; // ms
window.setTimeout(function() {
if (window[name]) {
callback(window[name]);
} else {
whenAvailable(name, callback);
}
}, interval);
}
And use it like this:
whenAvailable("twttr", function(t) {
// do something
});
The function given in the second argument to whenAvailable will not execute until twttr is defined on the global window object. You can do the same thing for FB.
Important note: Those libraries probably also provide some built-in way to execute code after they have loaded. You should look for such hooks in their respective documentation.