Here's what you may have missed this week:

• Apple Brings iBooks Textbooks And iTunes U Course Manager To New International Markets - Apple’s iBooks Textbooks and iTunes U Course Manager products are being launched in brand new markets in Asia, Latin America, Europe and beyond, bringing the total number of markets that iBooks Textbooks now reaches to 51, and the total for iTunes U’s Course Manager to 70.
• Facebook Teen Decline Is Real But Leveling Off - Facebook lost almost one third of its U.S. teen users in 2013, according to a newly released edition of one of the most comprehensive social media studies.
• iPhone Sales Broke 50 Million Mark Last Quarter - Apple is officially reporting its Q4 results on January 27. Meanwhile analysts have weighed in with their projections of sales during the period. There's not a huge amount of variation among the estimates: The range is 50 million to 59.73 million.

And some great stuff from great bloggers:

• Should You Take a Picture Right Now? By Daniel Zeevi
• The Smart Marketer Guide To Facebook's 15 Ad Types by Dennis Yu 

Is there something on this list that I missed? Let me know in the comments.