I work closely with an events agency in the UK, and have helped them manage the digital side of some massive events for clients like Samsung and Oracle.
I'm also an official TEDx organizer, so I know a bit about the non-digital side as well.
I've not come across many social media accounts or blogs that are platform-agnostic - The vast majority out there are product related and are always seeking to push the agenda and worldview of that particular product, business, agency or organization.
Considering that at this stage, I know nothing about the size of your event(s), the potential audience, your admissions/attendee model and ideal target income for your sponsorship budget - My advice to you would be to research events agencies in your local area, or in your audience space, that could help/advise you.
Here is the approach I would use:
1) Start small. In the events space, even small agencies can pack a big punch, and they are usually friendlier and more flexible.
2) Ask them for advice. If you're serious about organizing an event, they should be biting your hand off to advise you on what to do, and what to charge.
3) Offer them whatever you can to get the to buy-in. They might halve or waive their fees entirely if you can offer them an opportunity to speak, present or sponsor themselves.
4) Do a beauty parade. No matter how much you like an agency, always talk to more than one when you are planning and researching. You need to compare like for like.
5) Seek independent guidance. If you're new to the events space, then you run the risk of being taken advantage of. Once you have some options on the table, and you are looking at making a decision, run it past an independent advisor (like me) to get validation of what is being discussed.
I'd be happy to discuss this with you on a call, if that would be of benefit - I have a few more negotiating tactics up my sleeve that may prove useful.
Eventbrite is a popular social platform that allows event organization for free. If your event has ticket-related costs then they receive a small portion of profit off of the transactions that each attendee makes.
Meetup.com works a little differently. In order to create an event you must be a part of a group, but there is a cost associated with creating that group. I don't believe there is a limit on members of groups, but there is an application process each group has to filter members.