Back in 2009 I published a post on ERE.net "Call or Email or Use Social Media?" The post was about reaching out to potential candidates, or to business prospects, using Social Networks. While some of the technical details are, of course, outdated 5 years later, the idea remains - and you can do MUCH more now than back then.
I'd like to clearly identify the topic of this post. It is not about messaging your friends on Facebook and not about using "Social Messaging" apps on mobile devices. I'm going to go over messaging capabilities available for Social Network users, that may help to reach prospects - even if you do not know an email address or a phone number.
We know that some LinkedIn members send invitations to connect, carrying the messages to engage in business or apply for a job. I am not a fan of the approach; but if it's your cup of tea, you might want to check a recent post How to Connect if the Reason is Not Listed.
(Other methods to reach out to prospects, that I prefer to avoid, are direct-messaging on Twitter and messaging on Facebook; I will skip them in this post.)
On LinkedIn, you can send messages to your 1st level connections and to fellow group members. These messages go to the recipients' email address, registered on LinkedIn.
Since your business prospects are not likely to be your first level connections, your chance to use LinkedIn messaging remains with the groups. A member has a limit of 50 groups. Given the groups' population numbers, if you max out on groups (and if your prospects do tend to join LinkedIn groups), you might be able to reach roughly 1% of LinkedIn members via groups. That would be ~6 MLN people. If you want to message any of the remaining ~310 MLN members, who are outside of your groups, you would need to pay for InMails.
It's VERY different on Facebook! You can email pretty much any Facebook member who has the default preferences. I imagine that would be 99% of Facebook users (probably more than that). That would be ~2 BLN people. Note that, just like on LinkedIn, the message will go to their email address, registered with Facebook.
It's VERY different on Google-Plus as well. If you'd like to send a message to someone on Google-Plus, simply include the member in your circles, then use the "sharing" dialog and select that member (start typing their name to get a prompt). I am afraid I can't provide a good estimate but it could be that's another ~2 BLN people there as well, imagining that 99% members have the default settings. (I am not going to go into further detail on how Google+ accounts are tied with using Google, making Google-Plus population to seem very large. Even if you only look at active Google+ users, that would be a pretty large number as well: the number of active users monthly is about the same as as the whole LinkedIn Network has.)