#3 - like the font, but the plus sign is giving it a "medical/health/fitness" connotation. Also, this particular green shade isn't giving a powerful image. #2 - the sum sign within the circle is good idea, but not "web2.0". it's kind of something i would have used for an "offline" company. ours is going to be a universal online platform #1 - i like the font. it's comparatively better amongst the 3, but still not the best. I'm expecting something extraordinary.
PS: dear design gurus, please don't mind if i'm too direct in expressing my dislikes. :)
#4 - it looks good to the eye, but anyone who has seen "skype" logo will find an uncanny resemblance, if you know what i mean. though i like the web2.0 appeal in the logo. may be if you can try something else keeping the font and adding some mark.
#5 - i love the gradient effect, but the the circle image isn't that great.