From the faq I know that the following give XP:
- Crafting a badge ("most badges give 100 XP")
- Upgrading a badge (it looks the same as crafting a badge: mostly 100 XP)
- Years of Service badge (the faq is vague. What is the formula for XP gained?)
- "Special events on Steam also frequently have badges" (?)
What other activities give XP? Are there any ways to get XP other than getting badges? Do regular achievements for Steam games give XP?