That's a good question I guess you can find many websites with data about this topic. Check this site out and you wil be able to read about the most Interesting Uses of Camera Mobile Phones To Stay Productive

How many photos are taken on mobile phones? - Numerous number depending on the storage of your phone

What do people do with them? send to photoshare sites, email, MMS, keep them, share with people by showing them the phone, how is this changing? - Most people right now are sharing their photos on Facebook and other social medias using the internet.

Is the cost of MMS getting cheaper, thus making it easier to share photos? - Yes it is getting cheaper. Not really, 3g phones or internet capable phones are more easier to use to share photos since you can directly share your photos to your social medias.