使用PHP访问用户的facebook朋友的数字ID的奇怪差异

I've got a small problem. Using the Facebook graph for PHP, I access a user's info etc, from here

$user_profile = $facebook->api('/me?fields=id,first_name,last_name,email,gender,locale,picture');

Now, accessing the id from there will give me the regular numerical id, which I store in my database.

However, when I access a user's friends

$facebook->api('/me/taggable_friends');

And then access their id, it gives me a whole different form of id meaning I can't compare it to the ones in my database. Example: AaL4wTsOBmhG75qbHAWczJBaUZNlcxGr9C__pCEvgB1Y-oRPCpF2quaBw2oHrEU7-MKg3-5JYMNK_KR1C5rtz2r6Li1c65tfangz8JdLHIVt-w

What kind of id is this, where does it come from, and how can I reach the regular numerical id for the people in the friendlist?

Many thanks!