Like somewhere I could keep a diet blog or food diary or have some community like that.

Try www.layoffthedevildogs.com

sparkpeople.com
or if your a teenager
sparkteens.com
hope this helps!
You can track weight, keep blogs, talk to other members, and get free diet plans. :)

Here are a few.....
Free Diet Tips - Resource for weight loss and exercise information, ... Weightcheck - Online weight loss and exercise advice with a personal advisor for ...
www.dmoz.org/Health/Weight_Loss/ -
Free Weight Loss provides free weight loss info, products, diets, ... Online Resources Here are some of the best weight loss resources on the net! ...
www.freeweightloss.com/ -
Free online weight tracking diet program. Join Free Join Free! ... Our comprehensive weight tracking program allows you to plan weight loss goals and track ...
www.startyourdiet.com/

Despite the name: http://www.bodybuilding.com/ has a great community and many tools, tips, and workouts you can select from. You do not have to buy any supplements from them, you can simply click Enter Supersite instead.
Extrapounds is also a great one to use. It provides all of the things you specifically asked for.
http://www.extrapounds.com/
Almost forgot Sparkpeople.
All of these are free.