[Come-rad]

Very good points. If only my Lightroom could notice my fading interest and could say "Hey, pal. Grab your ass off the couch and go shoot a cityscape (you've enjoyed them). I'm challenging 5 other guys like you to make the same." and then show our photos each other.

[zimpenfish]

There's a few "weekend/topic challenge" groups/hashtags on Instagram and, if I recall correctly, there's a few iOS apps which are similarly "here's a theme, take photos, submit them" based.