I have been seeing a weaning out of the label follow us with social icons. Tell me if you think it adds value or doesn't affect the site. An example of a site that uses it is Mod Cloth. One that doesn't is Victoria secret.
I would split your question into two segments.
Is there any value of having inviting text (a verb) along with an icon. Simple answer is yes - having it is good than not.
What is the visual cost of having such phrase besides the icon as this may look untidy or design doesn't go too well with that. This is where you need to decide between cost and its benefit. At this point, my advice would be that instead of letting visual design control this aspect, let your marketing and user strategy decide.
I am of opinion that the term "Follow us" has lost its shine over the years and doesn't stand out or excite - even though it is understood well. Some possible alternates could be "Stay informed on twitter" or "Join the Pros" etc, but these phrases are rather longer and would require more space and even more appropriate positioning on the screen.
I hate to give the standard UX answer, but "it depends." It depends on what the goal of the site is. Do you want followers? Is that an important metric in some way? If yes, then you need them and you need them to be fairly obvious.
If not, I'd still recommend having them, if you think users might want to do that, but make them less obtrusive in that case.
If fan/follower acquisition is a metric you are tracking then you'll need to run some split testing and see how things go. Doing such will also allow you to test not only how the presence of the label or not affects acquisition but also if there's a better wording than "Follow us".
Like Phil said, it depends and the only best way to answer the question is to test and see how things go. Generally speaking, being more explicit is usually better; however, there are plenty of exceptions to this and thus, depends on your audience, site, and a wealth of other variables unique to your situation.
Adding a "Follow us" text next to a Facebook icon or button will help people understand that this is the link to the brands official Facebook account. If you only display a Facebook icon you will increase the risk of making people think that this is a "Share this page" link and ignore it.
But the real question is whether adding a Follow us phrase is enough to convert people into followers. It's like those bad banners that only states "Click here". If you could give people an incentive, a reason to follow you it's better. Think of why people should follow you. Is it because it's on Facebook you answer all customer service questions or is it a place where you can get coupons etc. Then write that instead.